Why is it okay when she wants sex, but I’m the creep when I do?

I'm 19, and my girlfriend and I have a pretty active sex life. We live together and are both university students here in Australia. We have equally high libidos, but I'm sometimes unsure how to initiate sex.

There are times when she's relaxed and things just happen naturally. But other times, when I try to initiate, she makes comments like, "All you think about is sex." I don’t want to come across as needy or pushy.

If she's in the mood, I’m always happy to go along with it, no complaints. But when it’s me initiating, she sometimes throws tantrums or makes me feel like I’m asking for too much. So now I feel like I have to think twice before even bringing it up.

For those who are married couples or people in the long term relationship here, how do you usually initiate sex with your partner without making it feel one sided or awkward?