Oooof - tricky!

OK - here goes. Firstly, your S/O should be and should remain the centre of your life. If you feel differently, then you are already letting her down and should address the problem one way or the other.

ANY other relationship you get into, M or F, will detract from what you owe your S/O - so don't start one. Don't even think about it.

With your S/O's participation, a 3some with a male who is not a close friend or relation is a possibility, as long as you realise that you have invited him into your bed so both you and your S/O can use him as a sort of super-sex-toy. Then he gets up and leaves, end of story.

Suggest you both look at another approach. Look at vixencreations.com or feeldoe.com and get a strap-on that fits into your lady and lets her fuck you with the other end (it's best to get one of these sophisticated ones that give her pleasure too). This way she can dominate your fem side and you will get your jollies within your existing relationship. Much better solution.