What do you do if your lesbian girlfriend cheats with a guy get pregnant and wants to raise the baby with you?

Ouch! The key Word here is "cheats" -- the reason isn't important (nor is the sex of the person she cheated with). Maybe even more important, the decision to have a child is one of the most important any couple faces. It sounds like she made that decision without you. She's put some pretty serious dents in the trust that is fundamental to any relationship. The two of you need to trust each other if you're going to raise a child. You're the only one who can say if you will be able to trust her any more. If you can't, it is probably best to end the relationship. Oh yeah -- get yourself tested for STDs -- she slept with another person, remember.