messed up feelings

Why is it that guys can have sex and not develop feelings  of like being "in love" but when girls have sex we tend to fall "in love".  It's not right, and its not fair and it totally screws up with your emotions. 

Lately I've been getting jealous of my friend and his friendship with another person. I start feeling like I'm not wanted when I find out that they got to hang out and I wasn't invited.  I wish I could stop these feelings from showing up in me. Its with this friend of his only that it bothers me. 

I just want to check out and forget about life if these feelings won't leave me alone.