I don't really care what other people are doing (LOL) in their spare time. If anything, I like the thought that they are doing something which won't advance them in any way that I consider meaningful (I'm quite competitive).
However, I wouldn't marry a girl who had had one night stands. I probably wouldn't marry one who had had sex with someone else to be honest. I realise this might seem unfair (especially to the non-virgins reading this), but I haven't had sex with anyone (not because I haven't had chances. some girls have liked me in the past and others have tried to set me up with their attractive friends, one time because the friend was concerned that she was still a virgin at 18), and I have plenty of acceptable reasons for not wanting to marry a girl who has.
Also, "you only have your youth once" doesn't motivate me to have one night stands or anything else which is just unproductive pleasure. When I think back to primary school, I don't really regret that I behaved how I wanted so didn't get invited to many parties, since the fun of the party would be over now anyway, and when I think back to high school, I don't regret not going to any parties I was invited to, again because the fun would be over now anyway. What I regret is the things I didn't do which would benefit me now, like taking more exercise, studying harder earlier on, getting into certain hobbies earlier, etc. When you're old, you'll still be old regardless of how much sex you've had.