im litteraly the worst gf ever😭

so im in college, im 19 and there is this really cute, tall Algerian dude in my class who is in the same team as me with 2 of his guy friends as well. they are the only three people i talk to in that one class cause it all started when I skipped the class once and it turned out there was a team work, so the next class I had no team, and they welcomed me so nicely in their team. then since monday (we have class 2 days a week), he would always sit next to me in class and talk to me and stuff, not flirt tho. but were not close. and one day he wore joggings or baggy pants idk and you could just see his dick print through it you know...and in my opinion it didn't seem small at all. and ever since that day I always fantasize of having sex with him cause he's muscly and super tall and just. cute. even tho I'm in a 4 month relationship and it couldn't be any better cause we're just doing so great. and i dont have the intention to cheat on my bf at all. but is it considered betrayal to fantasize of having sex with someone if youre already in a relationship? i know ill get rude comments but whatever, i know im not cheating on my bf. i wouldn't have the guts to do that since I've met his family and I truly love him