Why is everything on his terms?

I'm seeing a guy and have noticed that everything but mostly the sexual and romantic stuff is on his terms. For example, when he feels like flirting he will text something sexy/flirty and I play along because I enjoy it a lot. When I feel like it and he doesn't, he'll just ignore my messages or he'll say Thanks!
Because of it, I feel sometimes like we are just friends and nothing more even though we got together for sexual/ romantic reasons.
How should I feel about it? Should I just relax and blame it on his moods or it this something I need to worry about?
Why is everything on his terms?
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