If men and women have no specific roles in marriage such that the woman is to have certain responsibilities because she is the female and the man is to have certain responsibilities because he is the male, then why do you need one of each sex? Some might say that you need a male and female to produce children, but fewer and fewer see any reason why marriage and children are connected. If we accept that a marriage need not be open to children, then why does the ability to create children form a requirement for marriage?
It is also very notable that every church denomination that has accepted homosexuality has first adopted egalitarianism and female pastors and they use the same arguments for both positions. If "there is no male or female in Christ" means men and women are interchangeable and have no difference in roles, but should be treated identically in every respect, then that means we can't tell the difference between men and women when it comes to what marriage is either.
The two positions are logically connected, not because egalitarianism entails acceptance for homosexuality, but because acceptance of homosexuality entails egalitarianism. If you accept homosexuality as normal and healthy, then you necessarily accept egalitarianism.
Egalitarianism is the premise that men and women are not complementary, or at least that they don't need to be - that there is no design such that men and women are fundamentally different in complementary ways. There's no requirement that a healthy relationship requires a woman who is feminine and a man who is masculine under this view. Egalitarianism claims that the man and woman can act exactly the same, with the same responsibilities as one another, or that the woman could hold the masculine roles while the man holds the feminine roles. It doesn't matter how they do it, in this view. There's no proper roles. They don't have to complement one another. A union of two sames is just as normal and good.
Homosexuality is a further step that claims even their bodies need not be male and female. But you have to accept that there is no complementary design of males and females in order to accept that a healthy relationship could include two people of the same sex. You have to start with egalitarianism in order to get to acceptance of homosexuality. Egalitarianism lowers the activation energy necessary to accept homosexuality, if you will (to use a chemistry analogy).
If you believe that males and females are complementary by design, each having attributes throughout their design that provide something the other needs and lacks, then homosexuality is dead in the water. It could not possibly be healthy or proper. But you're already a lot closer to accepting homosexuality as a valid and healthy type of relationship if you don't see any reason that men and women have a complementary design that shows us their proper roles in the relationship.