Ideally, a healthy marriage is one based on love, children, and life-long support. But, realistically, sexual compatibility is still very important to most marriages. Sexual incompatibility is one of the top 3 reasons for divorce (the other 2 being money and infidelity). Because of this, I would never recommend a gay person to enter a heterosexual marriage. To me, it wouldn’t matter that the other spouse had full knowledge going into the marriage, or that both people understood that sex would be an issue. The fact that they are sexually incompatible will cause much frustration for both of them. The gay partner will not get the kind of sex that he/she desires, and the straight partner will know that the other one is unhappy (and who can be happy with such a relationship)? This kind of marriage has been practiced many, many times in our society, and there are a lot of divorces caused by it.
Theoretically, it works and could be considered a good marriage by Catholic standards. But it is my opinion that this kind of marriage will seldom work in the long run.