I always remember some advice TruckerPete gave to a guy with the same problem in another thread:
Originally Posted by TruckerPete
I have a friend who can't get hard the first few times with any woman he feels a connection to.
If you don't already have an open dialogue about the issue you're experiencing... I'd suggest you start talking about it. Something simple...
Also, how are you putting your condom on? Do you try to get it on as soon as possible when you know it's "go" time? That could be scaring your man parts. Try putting the condom on and not jumping straight to sex. Touch yourself or have her touch you with it on. Don't be afraid to go through a few condoms before you actually have sex.
I have also seen others recommend that a guy practice getting comfortable with wearing a condom by masturbating with one on as much as possible.
It's very soon for fluid-bonding. You've only known him six months. Furthermore, it is a boundary in his primary relationship which should be respected. You almost sound like you're having a tantrum about it. How would you feel if your husband had a gf who was harping on him to change a boundary you and he agreed upon, and which was very important to you? It doesn't matter what her reason is, just because you don't like how the sex is going doesn't mean you're more entitled to going bareback than she is. Where is your empathy for what she is going through, being mono and allowing space in their relationship for him to have a gf? I think her position as his mono wife -- who is obviously struggling -- should be given compassionate consideration for the gift she has given you both.
I think you should try every option for you and he to handle issues within your relationship without trying to get her to change for you by giving up a boundary before she's ready. It's an unfair expectation to place on her and, let's face it, your relationship with him is still in the beginning stages. And it's not her fault you can't talk to him about not being satisfied. Better off to try and work past that one and make the necessary effort to communicate honestly with him about your disappointment (without blaming him or her). If you love him, you owe him that much.
And an STD-free report from a few months ago is apparently still inconclusive. It doesn't mean there was no infection immediately after the tests were done. From what I've read, it needs to be followed up with another 3-6 months later, for more peace of mind.