People who don't want to live forever are basically saying that at some point - and they don't take responsibility for when - not only are they going to leave you but they WANT to leave you (because if they accept death as inevitable, then no matter how much they may pursue longevity or good health, they're choosing to die rather than do whatever is necessary to live). And I can pretty much guarantee that whether they're consciously aware of it or not, they'll want you to die too. Now, why would I want to become attached to someone when I know they want to leave me and want me to die? If I said to you, "I'm going to be with you totally and exclusively for two weeks and then I'm going to leave you", how intimate do you think you'd be willing to be with me for those two weeks? How about if I said, " . . . for four years . . .". or, " . . . for ten years . . ."? Why would it be any different if I said, "I'm going to be with you totally and exclusively for the rest of my life and then I'm going to die"? How can you really feel free to give your all to someone under any circumstance where there's a clear end in sight? This is the illusion of love and relationships. That because two people love each other, that that love is going to make everything else worthwhile. We delude ourselves into believing that the fact that we're going to leave each other is less important than the fact that we're with each other right now. How little we think of ourselves, how little we think of each other, when a few moments or even years of possible happiness take precedence over each other's lives.
Having lived the last twenty years interacting with immortal people whose priority is me and them living forever, without limitations, my criteria for being with someone in an intimate, sexual living are completely different. Physical attraction is still a factor, but unless there's a committment to the expansion of our aliveness and intimacy together, a passion for a greater living, not just for ourselves but for others, and an opening to more and more people in our lives, then our intimacy is limited, our togetherness has an end, and physical attraction is irrelevant.
For me, the whole point of being with someone is to have something greater moment by moment with each other than you would have if you weren't together. That to me is real intimacy. And if you have that then there'd be no reason not to be together, there'd be no reason to stop loving each other and there'd be no reason to want it to end. This doesn't necessarily mean that you ever have to sleep together or have sex with each other, or that if you did enter into such an intimacy together that it would continue, but if you're having "something greater moment by moment with each other" why wouldn't it? What it does mean though is that you'd never have to be without each other. That's what I call commitment. And that's where I find true joy is.
Why wouldn't I want that with the people I love - not for just a few years, but forever? That's the quality of togetherness I have with my 'friends' and that's the quality of togetherness I require in a sexual relationship.