Some guys like to penetrate, some others like to be penetrated. Some are vers (versatile) and like both but may have a preference.

If you're vers, there's no rule. You can be top and bottom during the same sex session if your partner is vers. Or just be either top or bottom for this session. If your partner is exclusively top or bottom, you can adapt to the situation.

About orgasms. You can have a prostatic stimulation, but the main point is that the anus is a very innervated area, so you can have pleasure just by stimulating it (e.g. annulingus). There's also a form of intellectual pleasure, and the joy to give pleasure to your partner.

And you don't have to get an orgasm each time you have sex. And if you stll want an orgasm, your partner can finish you off with a handjob/blowjob when he's done.

Imagine a world where straight sex would be exclusively oral sex. You could like getting and/or giving oral sex, or both (top, bottom, vers). You could have pleasure just giving oral sex, without always focusing on your orgasm.

Does the guy getting fucked in the ass always maintain that role? Or do the partners sometimes switch off? How does the guy getting ass fucked get satisfied with an orgasim ?