I'm the side chick or am I?

Here it goes:

First, I need to give you a little synopsis of my relationship with **Brian. Brian and I dated a long time ago during high school. I believe we started dating freshman year. We had a good relationship, he was caring, sweet, attentive, charming, and of course, very cute. ;) He was a jock and I, myself, also played sports. He was (and still is) very fit. I mean like, cut. Ufff! Anyways, this made him a real catch, so many girls would always flirt with him etc, etc. Long story short, I ended up breaking up with him because I found out he was 'talking' to another girl. I was 15 years old at this point and Brian and I never had sex, so the break up for me wasn't that bad. I was hurt of course but nothing major. This made it really easy to move on and just be friends. 

Fast forward to the beginning of senior year, I was freshly legal (18) and hadn't yet experienced any sexual activity. *Yes a real saint haha* But at this point in my life my interest had piqued and I was scared and nervous. My body had completely blossomed, DDs and a nice booty! (THANKS MOM!!!) So I often got nasty sexual texts/pictures/dms from thirsty boys. However, no one ever seemed like the right person to do it with. I got into a relationship with a boy at the end of my junior year, his name was **Liam. Liam was very patient with me when it came to the topic of sex. He knew I still didn't feel ready and I was scared so he never forced himself on me. We made out and did other sexual things. ( I must add that to this day I have not given anyone head. *Oral sex* This is due to issues I have) Anyways, Liam and I were very on and off and would often take breaks in our relationship. This is why I never felt like I wanted him to be my first. 

So back to the middle of senior year: Liam and I took a short week break because we were just fighting a lot. I was very hurt, sad, angry, and quite done with him. I wasn't sure where my heart was but I was also very lonely. One day Brian texted me and we started talking. He invited me over his house one day and I went. I didn't think anything of it. I got there and we just hung out. I had taken a shower and only shaved my legs, I didn't shave anything else because I didn't expect anything else. But one thing led to another and it became very hot and very heavy quickly. He reached under my leggings and didn't stop him. Even though if it had beef anyone else I would have since I was very weird about my self image and I hadn't shaved, but I was ready. I was comfortable and I wanted him. He made me feel like a goddess, and he pleasured me. But before he went in, as he was on top of me ready, he asked me this: "Is this your first time?" And for some odd reason, I said, "Are you kidding? No." I lied. Why didn't I just tell him the truth? Well he believed me because as it turns out I was "a great fuck." (?Boy language¿) I went home after that, took a shower, and felt great. A week later I was back with Liam and Brian and I stopped talking. He got a girlfriend and I had my boyfriend. Fast forward to December of 2017. We both were in our freshman year of college. I was still with Liam. (high school sweet hearts) *sike* And yes, we were still on and off. Eventually we both got tired of it and finally broke things off. Man was I depressed. I felt ugly, I started to just not be myself. Brian came home from college! He was back in town. We made plans to hang out and of course that turned into sex. Again and again and again.  But he made me feel wanted, he made me feel like I was myself again. So, Every time he would come back into town we would hook up. And I was fine with just being fuck buddies since I had gotten out of that relationship. Well after about 6 months of this I found out he had a girlfriend all this time that we were hooking up. Yup. I was the side chick... but I wasn't hurt, nor sad nor mad. In fact, it made me feel powerful, sexy, and mischievous! I felt like Beyoncé honestly. But we text often and he's always flirting with me. To this day we still hook up on the regular and yes, he's still with her. I still don't feel bad but I just want to know should I end it? Should I keep doing this even though now I KNOW he has a girlfriend. I don't have feelings for him at all, it's just sex. And again, I'm very okay with that. But I do feel like I, being a woman, should respect the girl even though I do not know her. However, Brian is so attentive with me. He pleases me without asking and so I am confused. Why is he still with her but he's always coming home to me. Why does he flirt with me and say things that make it seem like he wants to be with me. What should I do?  

Please do not judge me or call me a slut because I have only had sex with Brian. I have yet to find someone who I want to have sex with again. I seriously need help, maybe if you have gone through something like this or maybe you were the girlfriend that had no idea. What advice would you give me?