I have a feeling but I'm too scared to act on it.
Why are you scared? What's going on?
A guy I'm interested in has a girlfriend who is really strict with him.
It's okay to be afraid of things you aren't sure about. You should make sure you are right before asking the guy out.
Nah, I'm going to be a bad girl and ruin his relationship.
It is pretty bad to ruin someones relationship. You have no idea what their real relationship dynamic is and should find someone single to date or wait till they break up.