Well, unless the artist wanted to include an element of polygamy or multiple simultaneous partners into the story, "There Can Be Only One" winner of the contest for who gets hooked up.

Sometimes it's like that.

Yeah, well, when you see both sides of the story, that's just how it goes.
I mean we even get more background here on perhaps the moment where Takeshi first came to like her, perhaps. They had a little chat together on the beach while everyone else swam. The fact she thought about this little episode while on a 'beach date' with Senpai might be telling that she did in some way like Takeshi even while dating Senpai (Which helps solidify that theory in the sequel).
She's clearly happy being with her Senpai as shown in this release, while poor Takeshi who also liked her is just...out of luck. I do feel bad for him, but it's not like her relationship with Senpai is toxic as far as I can tell (though there are a few...implications in the sequel that the relationship might be heading in an unhealthy direction), so on one hand it's nice to see her getting along well with her lover, knowing what happened in the sequel chapter makes you feel bad for Takeshi, the guy in her flashback.
I like this release because in a way it helps give more...context? To the relationships between the characters in the sequel.

Yup.

A certain other website called this NTR, but I'm not sure it fits that bill.

Hmm.. So is she the same female mc??