I'm an adolescent teen boy and I get erections at random times, but then I got one when I saw a muscular man. I really like girls and I think they're hot and I want to have sex one day. Why did I get an erection?

Because you are an adolescent teen boy and get erections at random times.You could get an erection looking at a potted plant or a math book.Your body is just trying out all it's fancy new functions and apps. You have some of them in the beta, so there is a lot of try outs and adjustments.

Why do men moan during ejaculation? Is ejaculation too painful like menstruation?

Trust me. It's not a feeling of pain but a very, very good feeling of pleasure. That's why we moan. It feels so good that we can't think of anything else to do but moan. It's a sensation that's very hard to explain