I don't agree, he is a young guy, his hormones are pretty much still in overdrive, naturally he would react little understanding if his gf doesn't want to have sex with him after several months, outside of tv shows that's not really how most relationships works.
And like Ellaraia pointed out, if Lucy talked to him about the reasons, instead of just being snappish to him, whenever he asks, he'd probably be less annoying about it.
And don't forget, the first thing we see him do is eat his gf out, WITHOUT getting anything back for it.
That's definitely not being an ass to her. She is the one missing dates without giving a reason AND leaving him to clean up "shit" they produced together alone.