Do five precepts protect?

In the threefold training, sila (morality) corresponds to the precepts. Sila conditions samadhi (concentration) which in turn conditions panna (wisdom).

A common theme across the Buddha's teachings is that of nekkhamma (renunciation and pleasures borne of it). The precepts develop the quality of nekkhamma since the individual is practicing giving up certain unskillful qualities. When living by the precepts correctly the individual is practicing the following: abandoning arisen unskillful qualities, guarding against unarisen unskillful qualities, arousing unarisen wholesome qualities, and maintaining/cultivating arisen wholesome qualities. These four cases are known as cattaro-sammappadhana (four right exertions) which are associated with the path factor samma vayama (right effort).

One can meditate all they want but if sila is broken, i.e. the individual is deficient in virtue, then any samadhi attained is also going to be broken. Broken in the sense that it won't be samma samadhi (right concentration), so it wouldn't be conducive to attaining wisdom. From the perspective of the noble eightfold path, the path factors condition each other successively. Samma ditti (right view) conditions all other path factors, which is why it's listed as the first path factor, so it's not possible to have samma samadhi without first having samma ditti. And it's not possible to have samma ditti without being compassionate towards oneself and others since true wisdom goes hand in hand with compassion.

The training rules (precepts) are not hard and fast rules intended as prescriptive laws. Individuals who are advanced in virtue would go above and beyond the minimum criteria specified in the rules. For example, after having experienced peace of mind as a consequence of abstaining from killing, individuals are inclined to also give up any other type of physical or psychological harm. It's a positive feedback loop that leads to the abandonment of the second hindrance (nivarana) which is ill-will (vyapada). In the case of the third precept, a beginning practitioner will abstain from non-consensual sex. One who is slightly more advanced might give up visiting prostitutes. One who is much more advanced would give up p**n and masturbation because they see through practice that doing so is conducive to their meditation practice. The positive feedback in this case is conducive to the abandonment of the first hindrance which is sensual desire (kamachhanda).

With respect to rebirth in the lower realms, there is no guarantee that you will not be reborn in the lower realms unless you have attained stream entry. Practicing the five precepts can reduce the risk of being born in the lower realms, but understand that this is only temporary. A lot of Buddhists that believe in literal rebirth are very miopic about this. They think that making merit in this lifetime will help them secure a good birth in the next life. However, they fail to see that once that merit runs out they fall back down even lower than where they are now.

The precepts are generally considered to be training principles (sikkhapada) which prepare us for more rigorous practice or which generate merit in order to gain a more fortunate rebirth (sugati) in the next life. And by "more fortunate" we definitely mean circumstances in which one meets and can practice the Dharma.

Many Pāḷi suttas outline how sīla (practising the precepts) leads onto more intensive practices such as saṃvara "restraint with the respect to the senses" or indriyesu guttdvara "guarding the doors of the senses". And these in turn lead into more intensive practises like eliminating the hindrances, and then into practising jhāna and vipassanā. Of course in other schools of Buddhism the progression into more intensive practice follows different paths, but on the whole the precepts are seen as preliminary to practising more intensively.

Traditionally for lay people, who do not undertake intensive religious practices, practising the precepts is an important source of merit (puñña) which they hope will accumulate enough to crate a fortunate rebirth next time. Thus traditionally we could say that the precepts protect one from a bad destination (duggati). Traditionally making gifts to monastics and supporting their lifestyle is an important source of merit. Generosity generally is seen as generating merit.

In modern times the distinction between monastics and lay people has begun to break down. Some groups see monasticism as far less important that it has been traditionally, or indeed as irrelevant, and many lay people have now taken up practices traditionally only done by monastics. So it's not always clear how the tradition applies.

On the issue of visiting prostitutes I would say that if one has made a commitment to be faithful to a sexual partner, such as one does in marriage, then visiting a prostitute is clearly a breach of that commitment. It is a lie, and therefore a breach of the precepts.

If one has no such commitment then the situation is more ambiguous. But in order for the act to be completely ethical one would need to know for sure, for example, that the prostitute was not under any compulsion whatever, to do sex work. One would need to be sure that the agreed fee was absolutely fair and that the prostitute received their full share of that money. In my view one could never be certain of these, and thus one would always be risking demerit or apuñña.

As with all the precepts, one must look at the spirit of them as well as the letter. The fundamental principle is "do no harm". One cannot choose to do a harmful action and claim that it is not mentioned in the letter of the precepts. Harm is harm. The institution of prostitution almost always does harm to someone. One can always ask oneself the question: "Would I want my son or daughter to earn their living having sex with strangers for money, risking catching AIDS and so on? Is that what I would want for my children?" My answer to this, despite not having children, is "no".