Simple question - what level of hotness is required for it to be worth sticking with one woman, and passing up the ability to play the field?
If you think "hotness" is the principal feature that will keep you monogamous, then, one way or another, I think you are in for a surprise in your future.
It ain't about the woman, it's about you. Show me a smoking hot babe and I'll show you a guy that's grown tired of screwing her 6 months later. Sex, while important, should not be the determining factor in the decision to develop a long term relationship. Full disclosure: At my age, now easier to do. 30-40 years ago...impossible, which is why I never married until I felt I'd had my share, and then some. One of the few good decisions I made while young.
Can you clarify what you mean, Gabfly? IMO sexual attraction is the only thing that distinguishes friendship/platonic love from romantic love. If mere personality counted, then we'd all marry our best friends and never have sex with them. Not only does no one do this, but pretty much everyone would find the concept either ridiculous or repulsive. So strong sexual attraction is necessary for any romantic relationship to work properly. You don't have to take my word for it, just ask anyone in a sexless marriage, or any of the 50% of marriages where one partner had an affair. I'm assuming that you get along with your woman well in the first place, obviously if you hate her guts then you won't stay even with Aphrodite herself.
But surely you agree a sexless relationship can't work? So you need to have mutual attraction strong enough that both people are sexually satisfied. Also, having lots of great sex with someone brings you closer to them emotionally; being sexually frustrated with someone drives you apart. I probably used the wrong term with 'hotness', I don't just mean looks, but also physical/sexual chemistry. You might be dating someone with average looks, but if she has a certain appearance and mannerism that make you want to do naughty things to her, then that could make you more attracted to her than to a supermodel who is a haughty cold fish.
Truth is: Eventually you will be bored of her (sex, looks, personality, chemistry etc) - and she bored of you. People are only as promiscuous as their options.