Strictly speaking, only men actually having sex with men is illegal.
Technically speaking, only sodomy is illegal. Which applies to both men and women.
If you get caught, it's because you're in public (part of public indecency, regardless of gender and act you were caught performing).
The government has stated explicitly that they're not going to remove the law, but they're not going to enforce it. It's really exasperating, because they're just appeasing both liberal and conservative Singaporeans. If the police starts crashing people's parties, then I would think they have nothing better to do.