In a committed partnership or marriage where one partner is straight and the other is gay or bi, the term "mixed orientation marriage" or partnership is used.
The best known form of mixed orientation marriage is where the woman is straight and the man is gay or bi, and there are many support groups for this situation (google it or check wikipedia). Arrangements where the man is straight and the woman is bi/gay are considered rare, but this is only because those arrangements are not reported as often. Draw your own conclusions on that point.
Mixed orientation marriages often end in divorce, unless a mutually beneficial solution is found. Polyamory is often cited by psychiatrists and marriage counselors as a healthy way to solve the problem.
Hope that helps.