Yes, if you only cared about what happens tonight, then a one-night stand would seem to make sense to you. But if you're thinking about the rest of your life, then a one night stand doesn't make any sense -- at best it's a waste of your precious time.

But if you're not a utilitarian/pragmatist, then your time-frame is morally irrelevant -- you will decide your actions based on the nature of the actions you are considering.