Good questions, and it's understandable that many people might associate anal sex with gay sex. But the ass has a lot of pleasures for straight people too. I can speak as one who identified as straight for a long time and I've fucked women in the ass.
For women: many (or I suspect, most) don't start out wanting to get fucked in the ass. Many who try it are coaxed or pressured to try in by their partner. Some might be curious about it (heard about it from porn or female friends) and might be wondering what it's like. Some who try it, like it. Many might hate it for all the obvious reasons. My partner once said "it feels really good" but I knew she had to be extremely turned on, and it wasn't allowed to happen most times we had sex. I think the physical side of anal sex is just as intense and pleasurable (when done right) for women as it is for men, and I enjoy receiving it so I understand entirely. Some might enjoy the physical feeling, but might have more complex feelings about other downsides like potential for pain, mess, or feeling degraded. I've heard of women complaining that once they give their partner anal, he doesn't want anything else, and then the sex can feel like a chore. Women are complicated sexual beings - their stimulus zones are on the outside, and if they feel like they're just a life support system for holes for a man to use to masturbate in, then they're not going to enjoy sex. Anal sex for a woman (and I think I was doing it well for her) can be a pleasurable thing if it's mixed with a whole package of care and attention, but not (I assume for most) if it's done as portrayed in porn.
For straight men: curiosity and FOMO play a bit part. It's so commonly portrayed in porn etc. that men feel like they're missing something if they haven't had it. There's an idea that it will be tighter or feel better in some way. It's a "final frontier" and there's a challenge and prestige in getting a woman to allow him to go there. It signals a lot of trust and submission on the part of the woman that can make them feel important, proud or loved. I have to admit, the first time I slid my cock into her ass I thought WOW! I've finally arrived!
For my personal experience in a straight relationship, I've tried it, we enjoyed it, I was overjoyed that she trusted me enough to let me do it and that I could make it feel good for her. But having tried it, I'm happier in her pussy for a whole bunch of reasons. Hope that answers your question!
