Can this cause a rebirth as a homosexual?

Possible, on causes and effects, see answer in related question: What makes someone become a gay? , good householder.

So now good householder goes on to watch more Mr. Bean, or else? Uposatha Silas are of much benefits, countless more, for long time happiness.

Actors perform, intoxicated with the three poisons i.e. passion (also lust, greed), aversion (also hatred) and delusion, and they do so, spreading it to their audience.

Then the audience too become intoxicated with passion (also lust, greed), aversion (also hatred) and delusion, and this will affect their future, naturally, because what one consistently thinks and ponders about, becomes the inclination of their awareness.

This applies to any movie or performance, with unwholesome themes. The unwholesome theme in this case is passion or lust (lobha).

"Apparently, headman, I haven't been able to get past you by saying, 'Enough, headman, put that aside. Don't ask me that.' So I will simply answer you. Any beings who are not devoid of passion to begin with, who are bound by the bond of passion, focus with even more passion on things inspiring passion presented by an actor on stage in the midst of a festival. Any beings who are not devoid of aversion to begin with, who are bound by the bond of aversion, focus with even more aversion on things inspiring aversion presented by an actor on stage in the midst of a festival. Any beings who are not devoid of delusion to begin with, who are bound by the bond of delusion, focus with even more delusion on things inspiring delusion presented by an actor on stage in the midst of a festival.

Thus the actor — himself intoxicated & heedless, having made others intoxicated & heedless — with the breakup of the body, after death, is reborn in what is called the hell of laughter. But if he holds such a view as this: 'When an actor on the stage, in the midst of a festival, makes people laugh & gives them delight with his imitation of reality, then with the breakup of the body, after death, he is reborn in the company of the laughing devas,' that is his wrong view. Now, there are two destinations for a person with wrong view, I tell you: either hell or the animal womb."
SN 42.2

"Whatever a monk keeps pursuing with his thinking & pondering, that becomes the inclination of his awareness. If a monk keeps pursuing thinking imbued with sensuality, abandoning thinking imbued with renunciation, his mind is bent by that thinking imbued with sensuality. If a monk keeps pursuing thinking imbued with ill will, abandoning thinking imbued with non-ill will, his mind is bent by that thinking imbued with ill will. If a monk keeps pursuing thinking imbued with harmfulness, abandoning thinking imbued with harmlessness, his mind is bent by that thinking imbued with harmfulness.
MN 19

Also, please see MN 57 for the story of ascetics who pretended to be animals, and SN 42.3 for the story of the warrior in battle.

Since there is no normative difference between gay and straight from either a biological or a Buddhist perspective, why does it matter?

Sexual orientation is caused by a complex interplay of genetic, hormonal, and environmental influences. It's not a conscious choice. So, in the same line of reasoning, if you watch a movie where you feel sympathy for a character suffering under genetic, hormonal or environmental factors, like:

• Sophie's Choice, you will be worried to be reincarnated as a persecuted woman?

• a documentary on Stephen Hawking, you will be worried to be reincarnated as a man with Amyotrophic Lateral Sclerosis?

• a documentary on bone cancer in newborns, you will be worried to be reincarnated as a baby with bone cancer?

• a documentary on HIV/aids ravaging populations, you will be worried to be reincarnated having HIV?

• a documentary on tsunami victims, you will be worried to be reincarnated to die in a tsunami?

If your answer on any of these is "no", I would suggest some introspection as to why you treat that condition differently.

For clarity: I'm not comparing sexual orientation to a disease. I'm giving examples of conditions caused by genetic, hormonal or environmental factors.