So let me sum this up:
She fucked a guy - unprotected - and also let him jizz inside her so that's about as intimate as it gets. Means she doesn't care about her own health or yours obviously. Then she came home and fucked you as well probably with the other guys stuff still inside her. That's just freaking evil and messed up.
She lied until you caught her and now she is telling you that it's your fault and sleeping in your bed peacefully while also threatening that she can do it again anytime just by walking over to his place.
You have been in this relationship only 18 months, have no kids and you own all of the property and assets?
This is a no brainer. Pack her shit up, throw it on the lawn and change the locks. Let her go over to his place for somewhere to live. But, like the other guys have said, have your ducks in a row first re:divorce lawyers and do it when she least suspects it. Preferably at 1am.
Also, make sure all of her friends and family know why you're throwing her out and if the other guy has a wife, tell her too.
Every time you think you can't do this, remember that time she fucked you knowing that she had another man's dick in her mouth and pussy hours before and that she was laughing at you when she did it.