Trouble understanding "sensual misconduct"

There are different gradients depending on your goal in Buddhism ranging from even sensual selectivity (a form of greed) being an evil (sin/error) all the way to consensual sexual promiscuity being quite alright! Read "The Six Dharma Gates to the Sublime" for information on the highly honorable former scenario. It is only meant for those aiming for bodhisattvahood or samadhi in this lifetime.

Read some pick up artist books for advice on the latter.

Across all traditions though, marital infidelity is considered wrong [although ultimately what does it matter because over millions of lifetimes how many vows of "love" are kept?].

This is a general moral construct created to help avoid bad karmas of many types--but not the bad karma of sexual suppression (that will eventually cause correlated rebirths).

The Pali text for third precept says:

Kāmesumicchācāra veramaṇī sikkhāpadaṃ samādiyāmi.

veramaṇī sikkhāpadaṃ samādiyāmi is a standard phrase that means "I undertake the training rule to abstain from ..."

The key therefore is Kāmesumicchācāra, or, parsed out, Kamesu-Micchacara.

Kam-e-su is usually translated as "sexual" and Miccha-cara as "misconduct".

Miccha, the antonym of samma, generally means "wrong", "low quality", "sloppy" and literally "opposite", "contrary". Cara means "conduct", or literally "going about", "walking".

Kama, the root of kamesu, is often translated "lust" or "sensual pleasure" but its more precise meaning is fondness (combining such elements as love, desire, and taking pleasure in) for an object of material senses -- in other words "sensual fondness" -- not sexual but sensual!

The key to understanding this phrase though is -e-su in Kam-e-su, which gives it the meaning of "out of".

A more precise translation IMO would be "a conduct incorrectly motivated by desire to partake of sensual pleasure".

So whether the act itself is of sexual nature or sensual nature or not at all, as long as it is motivated by desire to attain mere pleasure, it counts as sensual misconduct.

At the same time this means, even a forbidden sexual act should not count as misconduct if it pursues a higher goal - such as engaging in kamayoga for the sake of Enlightenment.

Not everyone interprets it this way though. This is a very "Mahayanistic", or should I say, "Vajrayanistic" way of seeing things.

As far as Buddhism and sila are concerned, "over-indulgence" is a bit redundant. To indulge is to gratify oneself with the object of desire; i.e. you desire chocolate, you then follow that craving and indulge in it.

There are obviously varying degrees to this type of behavior, but even one taste of the chocolate you desire, with an unwholesome state of mind, is acting against the spirit of the precepts that one aims to keep.

The precepts are kept as a solid foundation and basis for the meditation practice, helping to keep one in line with the basic behavior that is conducive to progress in the practice, and so a sense pleasure at any level could be considered misconduct, depending on the state of mind relating to the pleasure.

Starting out with the practice, you shouldn't beat yourself up over this stuff, because you WILL slip and mess up repeatedly. You will come to see through practice that these mess ups in keeping in line with your precepts actually do throw off your practice, and in this way you come to learn from your mistakes and see first hand the real value of keeping the precepts.

I hope this helps clear this up!

Since your question is tagged "5 precepts" I'm guessing you are referring to the 3rd precept?

It's generally translated from the Pali as:

3. Kamesu micchacara veramani sikkhapadam samadiyami

I undertake the precept to refrain from sexual misconduct.

source: http://www.accesstoinsight.org/ptf/dhamma/sila/pancasila.html

So while there are lots of considerations to indulging or overindulging in sense pleasures; the misconduct referred to in the precept is sexual misconduct.

For additional context for the interpretation of the 3rd precept as such is AN 8.39 which describes the 8 rewards of merit (one each for taking refuge in the Buddha, Dhamma and Sangha and one each for each precept) and the 5 great gifts associated with the precepts. It reads in part:

"Furthermore, abandoning illicit sex, the disciple of the noble ones abstains from illicit sex. In doing so, he gives freedom from danger, freedom from animosity, freedom from oppression to limitless numbers of beings. In giving freedom from danger, freedom from animosity, freedom from oppression to limitless numbers of beings, he gains a share in limitless freedom from danger, freedom from animosity, and freedom from oppression. This is the third gift, the third great gift... and this is the sixth reward of merit...

Is indulgence in any sense pleasure at any level considered a form of misconduct?

Whether or not sensual indulgence can be regarded as misconduct we have to examine it from the perspective of the Noble Eightfold Path.

In here we have the Concentration Group in which there is a factor called "Right Mindfulness".

Right mindfulness means to keep the mind in the present and not let it stray off into the past or future or into processes of ideation. Right mindfulness when practiced correctly allows for what we call "bare attention". This level of attention is free from any extrapolations, thinkings, imaginings, judgings, ideations and other methods of conceptual proliferation that the mind can make use of.

Right mindfulness lets the mind be with the object as it is. Objectivity and non-interference is present. The object is apprehended exactly as it is and one will be able to see it's true nature.

When one does not practice right mindfulness the mind will only for a brief moment apprehend the true nature of the object whereafter it begins a cognitive process of mental fabrication in order to try to grasp the object. The mind will take the "raw material" of the object and thereafter bring forth subjective concepts and ideas about the object.

The mind then weaves together all these different concepts into more complex structures. These are embellishments fabricated by the mind and does not belong to the object. It's the mind superimposing qualities onto the object. This process of fabrication is fueled by the underlying latent defilements. The end result is an object which is now covered with a thick layer of mind-generated ideas and qualities. The original object is now hidden in a dense layer of subjective artwork that hides it true nature.

Because the object is not viewed as it is but instead from a point of view controlled by the latent defilements the conduct has now turned into misconduct. It is not considered right conduct if the mind has strayed off into the past of future or begins to superimpose qualities onto the object. One then begins to identify with object and thinking that "I am seeing" or "I am thinking". This kind of belief is called personality-belief (Sakkayaditthi). "Sakkaya" means the group of materiality (rupa) and mentality (nama) and how they exist. "Ditthi" means a wrong view of belief, i.e. believing in a Self regarding Nama/Rupa.

Right conduct is to guard the mind and guard the sense doors so that the defilements are not running the show.