I agree that admitting it can be social suicide. But I think that's true only if it is the person's only social group; and it is approached either ham-handedly or if the group in question is relatively petty or immature.
I think it being mutual is the bigger danger. A group is more likely to survive a false start intact than a breakup. If it doesn't work out, people don't even really choose sides. They already know who is going to stay and who is going to go.
It's difficult to practice something where failure means disaster.
You can flirt with a large variety of people and the social consequences are pretty low. Thinking about how people respond to it is also free. If it's rejection you fear, say hi to strangers. Some of them will ignore you (or maybe even be bothered). That's rejection. It feels like rejection. But the stakes are so low it's safe and you'll become inoculated to a degree or at least comfortable with it. If you want to get better at asking people out, either roleplay with a friend or ask people out that you find interesting, even (especially) if you don't have a crush on them. Bonus: You might get practice at dating, or just find someone you really like.
I have tons of female friends in social groups whom I've asked out. Some had boyfriends or even husbands and we all had a good laugh about it afterwards.
Where it gets messy is when people start hanging out one on one with members of the opposite sex, calling it "friendship", and then confessing feelings.
And if it is not mutual, admitting it can be a total social suicide.