In casual speech, asking about what sounds more feminine or more masculine can be tricky. There are some things which are decidedly more manly or more feminine-sounding. Others are more ambiguous or are pretty much gender-neutral. You really have to hang around native speakers for a while and listen to how they talk in order to learn which phrases seem more masculine/feminine.
In my experience, I've heard male and female native Japanese use both 行こうか and やめてね.
That said, 行こうか seems pretty gender-neutral. As a male, I use it myself a lot, but it's perfectly normal to hear a female say it.
I'd say that やめてね is something that a girl is much more likely to say than a guy. I've heard mostly females use it. Although a couple times I have heard my college-age male friends use it, but in sort of a joking, whiny way.