Is having sexual activities with your SO while you're mad at her/him wrong?

We (M25 and F21) were at my gfs house when her parents went for a walk. We went to bed and after some making out I started to finger her. After few minutes she suddenly dressed up and said her parents are gonna be back any minute and left the room quickly. (obviously they didn't come back for another 30 minutes, but that's just a sidenote)

It was like 5th time in a row where I made her feel good and she didn't reciprocate (she does this a lot, just goes to sleep if I don't ask her directly for a handjob and she's afraid to have actual sex because we can only do it at her parents house and someone's always there).

I was really pissed off. I know sex is not about wanting something in return, but I felt really shitty. When I confronted her about it, she went into full shitstorm, how she and her parents are being used (I was living at her house for 2 weeks on vacation and frankly, I became a little apathetic and didn't do much around the house - she was being a little distant and so did I), I'm arrogant, childish, etc. and she wants to break up with me.

Now, am I arrogant or childish is another thing, but I wanted to ask you, do you think I have the right to feel used? Isn't having sex with someone in these circumstances kind of a manipulation?

PS When I asked her later, why does she rarely reciprocate, she said "I don't know, maybe I'm heartless"

Thanks.