take a look at this girl. she is a stunner. any guy would marry her, and support her, so she would never even have to work, let alone do porn to make a living.
I'm not sure choosing to do porn or not has anything to do with marriage or not. Not every girl that doesn't settle down goes into porn, some pornstars are married. Plus as a general rule, I don't think "get married and husband will support me" is an actual ambition held by any women with a grain of self-respect these days.
Why are there beautiful women in porn? Because porn is mostly made for straight guys, and most straight guys like fapping to good-looking gals.
Why do the beautiful women
get into porn?
(1) Sometimes a beautiful woman is making good money as a nude model and thinks "maybe I could do porn and make even more money."
(2) Sometimes a beautiful woman doesn't want to do a normal job or lacks the skills or qualifications to do the job she'd really want, but thinks people might pay to watch her fuck.
(3) Sometimes a beautiful woman is a completely insatiable nymphomaniac and rationalizes that if she's going to have as much sex as possible, she might at least try and get paid for it too.
Beautiful women have the same reasons to do porn as not-so-beautiful women. Sometimes they are unsavoury, I imagine - coerced into it, need money desperately for whatever reason, highly contrived revenge-on-the-cheating-ex plot (joke), but I like to think those ^^^ are the top ones. Unless you're desperate for the money I can't imagine anyone going through it if they didn't really love to fuck.