Maybe because she introduced Sena as her boyfriend to her parents? Meaning that she now has to find someone else to fill that role?
I'm guessing it's something like this. The crack about finding a "new
boyfriend" says to me that Sena is taking issue with Makino's sister's reluctance to be open about them.
The conflict between wanting to be open and wanting to be safe is a real struggle. It's hard enough to deal with when it's only about yourself.
Like, the only people
I've come out to irl are people I was 100% certain would be supportive, and it was still
insanely nerve-wracking to do it. It's great if it goes well, but no matter how certain you are, there's still a voice in the back of your mind saying "don't do this, it's too much of a risk."
edit: on a less depressing note, I'm calling it now: Mashiro only went to Sena's place to support her after her break-up. Makino
thinks she's there to hook up with Sena, but in reality she's just being a good friend.
At least... I hope that's the case...