For me, there's a misunderstanding from some people here. Keiko just isn't gay. She's not going to change orientation just because Ran is cute. People complain when the opposite happens (lesbian falling for guy) so why is this different?
There's no evidence Keiko went out with guys because she likes them so much either. There's a lot of social pressure in Japan to date guys and a lot of disincentive to date girls. From what we've seen, Keiko's relationships with guys have largely been unhealthy attempts to stave off loneliness, which doesn't necessarily mean she's straight in the first place (for instance). The whole "I tried to date guys and have sex with them, but it never really satisfied me" is a trope in yuri for a reason. Plus of course the possibility that Keiko is bi.
There's really not a single thing in Stretch that closes the door on Keiko having a relationship with Ran.
Edit: Far as I can tell, the only male Keiko actually fell for was her ex, and we know how that turned out. However, she thought Ran was cute the moment she saw her and has retained that feeling to the present day. That seems, given the subtlety with which most information in the series has been delivered, a strong suggestion that Keiko is, indeed, bisexual. And probably prefers females.
last edited at Oct 16, 2015 1:04PM