[Sabusuka] Ch.01 (COMIC MUJIN 2013-09 [Digital])
[Sabusuka] Ch.02 (End) (COMIC MUJIN 2013-11)

I've been reading this, so I'll summarize what I've read so far. The girlfriend, Minamo, is uncomfortable with perverted things, so she didn't allow the MC to do anything. After they go to the restaurant, they meet the MC's female sempai, who (I'm not 100% sure since I didn't read carefully enough, but I believe...) was his formor girlfriend/lover. What I am sure of is is that the MC liked the sempai at some point in the past (but he says truthfully that of course now Minamo is the girl he likes the most), the sempai was very assertive/overbearing/coercive and pushed him around in a sense and that everyday she "played around" (=sex obviously) with him. Anyways, this activates Minamo's "jealousy mode" and she starts asking him "what sort of things did you do with her" and she starts doing the same (presumably so that she can show that she's better than the previous girl). As you might expect, the MC makes some stuff up, and Minamo follows through.

I'm still reading it, so I'll add on to the summary later.

Base +5