Anxious over my sexuality, help please

I'm honestly having anxiety over this. I'm 19, have been in a straight relationship with a wonderful guy for 4 years now. I'm literally head over heels for him, love our relationship, wouldn't want to be with anyone else, and find him ridiculously attractive/sexy.

I have (pure) OCD. Around age 14 or 15 I remember having intrusive thoughts that gave me a lot of anxiety about girls. I was afraid I was lesbian/bi/attracted to girls. I am NOT homophobic. That died down a lot and I went pretty boy crazy until I found my current BF. I love having sex with him and all that. But recently my worries have started up again. Now I avoid looking at girls unless they're my friends because I'm scared I'll be attracted or something. (This is my OCD talking I think).

I get turned on by watching lesbian porn or watching girls masturbate and I'm so ashamed. I have no personal interest in being lesbian or bi. I have no romantic interest in girls at all, would never want to date one/marry one/etc. But I do find that I get turned on from that porn which scares me.

Here's the thing though, a few days ago, my boyfriend was eating me out and I started imagining that it was a girl and I came. I am so embarrassed and have no idea why that happened. It alarmed me so much and made me really upset. I also had two dreams recently with girls in them but I'm hoping it's just because I've been thinking about it so much and not an expression of my actual desires.

Now during sex I worry "am I enjoying this enough? if I'm not does that mean something?" Etc etc which isn't normal and is from my OCD but I feel like it holds me back from enjoying our sex.

I've always had some difficulty cumming with him (it takes 15-20 min) and that makes me worry even more.

I've never had a crush on a girl or anything. I wouldn't even want to kiss a girl, it's really just the sexual aspect that turns me on. I'm not interested in indulging in these thoughts any further and I just want them to stop.

This is seriously making me anxious. I want to marry a man and have a family with a man. I daydream of my wedding to a man.

I love the idea of being in relationships with guys, and definitely not with girls. So does that mean I'm straight? Why do I get turned on by the thought of sexual acts with girls then?

(Just a little side note, I'm in therapy for my OCD. Not interested in sharing this aspect of it with my therapist so here I am).