In my opinion, it is probably pretty hard for straight men, bc every woman is so different. With a penis, you more or less can do basically the same things and the guy is going to get off; with women, this is just not the case.
Sometimes even my own clit baffles me; I can masturbate for half an hour and still not achieve an orgasm; I swear the thing can be like the Sphinx! (I hate those times, so frustrating!)
And as far as penetrative sex goes, it can just be so variable. Some men I have had sex with have easily been able to make me cum the first time; others, it takes a while and several sessions before I am able to. And even I couldn't tell you why this is.
I personally feel like some people just "fit" better together; whether it's hormones, pheremones, or just downright the pure physicality of the 2 body parts fitting together better.
However, I also feel that part of it can have to do with how comfortable the lady is with her own body. I read so many articles about women not being able to achieve orgasm through sex. A lot of women are weird about their bodies and weird about their own sexuality. Bc sex is very cerebral for women, any kind of mental/emotional issues can also get in the way of them being able to relax and full enjoy sex.
My suggestion to you is to take your time with a partner, ask her for lots of feedback on what feels right and what doesn't. Spend time with a lady who is happy to let you explore.
Good luck to you sir.