[Private]
--In part. It depends on the person you're with, and it depends on you. How you feel, how you feel about them, what stage you're at in your life. All of these things are factors. Sex can be... very complicated, I'm afraid.
But here's something that's easy: Never do anything that makes you uncomfortable. If you're uncomfortable, then you aren't ready. The only thing you should feel, when it's really the right time, is -- excited, and a tiny bit nervous about trying something new.