Women fall in love with Gay guys all the time, just as straight men fall in love with Gay women. It could still work out, he could have realized she loved him - and then explained after they were married that he wasn't interested in women, but still loved her. There is a difference in loving someone, and being in love with them. As such he could arrange a sort of Heir situation with her, or at least attempt to. While still loving her and being a good husband to her. But perhaps they can start drifting as the story continues, and they petition King Thor to dissolve the marriage because they make good friends but not a married couple? You could start having them bickering over things that people normally wouldn't, because the pressure has finally just started coming to a head after all these years which is not uncommon in unbalanced emotion relationships (IE one partner is completely in love with the other, and the other just 'loves' them.). Then you can have him look into finding a Political wife, that perhaps could be gay as well - but wants children and is willing to marry him so she can have one, and he could have his partner to the side and she could have her own? That way they both have what they want - while putting on the face that the Kingdom Needs, and Natasha can return to Shield if that is what she wants?
Just... some thoughts... sorry.