I'm a lesbian with a bisexual girlfriend. Last night she told me she misses having sex with men. We've already tried a strap on but says it's not the same. She wants to go to a swing house to have sex with a man. What should I do?

If you are comfortable in having an open relationship, then I would say that is fine.  If you are not comfortable in having an open relationship, then let her know you are not okay with that.  It may be a deal breaker, but you need to be satisfied in your relationship as well.

(Not all bisexuals feel the need to have both genders at the same time.  Bisexuals can be monogamous.)