I want to clarify that the details I think I remember may be wrong.
The protagonists are a couple of high school or college students. The boy is pale, with straight black hair, and so is his girlfriend, who wears her hair in a ponytail. I think the antagonist studies with them and approaches the girl, giving her English or some subject classes at her house with a mutual friend. The latter may be wrong, but I know that the girl would get together with the antagonist and another girl in a house, and the antagonist, along with the girl, would convince the protagonist to have sex. Obviously, the boyfriend wasn't with them and didn't know what was going on either. I only remember that they tried to convince her to have sex and they succeed. I also remember a scene in which the boyfriend goes to look for her at that house without knowing anything, he enters the house and looks for her, but he can't find her, she was in the bathroom having sex with the antagonist while the boyfriend was near the door.
It is possible, although it may be wrong, that the antagonist had the typical bad boy appearance with brown skin and straight blonde hair, and a physique similar to that of the boyfriend. The manga has at least two chapters.
I think the drawing style is similar to Terasu Mc.