I hate it when my girlfriend asks for sex

You are not the only man in the world to feel this way. However, most confident, communicative, committed couples take turns, because if they didn’t, sex would never happen.
At the start of a relationship no one needs to think about “initiating” because life is a full-time sex fest, but by the time the lustometer has been restored to normal, it becomes rarer for couples to find themselves in the same mood, at the same time, under the same roof.