i) I think the most emotionally appealing argument for SSM is the claim lifelong celibacy is unrealistic. There are different components to this contention.
a) It seems "unfair" that heterosexuals can have their physical and emotional needs satisfied, but homosexuals are not allowed to do the same thing.
b) In addition is the empathy argument: If you put yourself in the shoes of a homosexual, how would you, as a straight man or woman, cope with lifelong celibacy? If that's not a realistic option for you, why would you inflict that on homosexuals?
I do think this is a challenge we need to address head-on.
ii) There are roughly two components to the issue: sex and companionship.
iii) Sexual activity is a matter of degree. Because the Bible forbids premarital sex, faithful Christians must be celibate before marriage. And that often means they must forgo sex for several years.
I'd add that postponing marriage into your 20s can be imprudent.
iv) You have a fraction of the population that desperately wants to marry, but can't. You have men and women who just aren't "eligible" by conventional standards. In some cases that's unfair, but that's the situation they're stuck with.
v) Widows and widowers often outlive their spouse by many years. And they frequently lose their spouse at a time of life when they are most vulnerable.
One reason they married in the first place was to have someone to grow old with. But now they find themselves alone at the very time of life they most dreaded that prospect.
vi) Then you have Christians like George Whitefield and John Wesley. Technically, they weren't celibate. However, they spent most of their time away from their wives. Mind you, I don't think that's commendable. They basically deserted their wives. Given how little time they put into the marriage, they should not have gotten married in the first place. But that's a separate issue.
My immediate point is that most of the time they were sexually inactive. Most of the time they lived like chaste bachelors. And I'm sure that's true for many single men and women who went into foreign missions.
vii) At the risk of stating the obvious, some married people are very lonely. They may find themselves in a loveless marriage.
viii) Likewise, you have "empty nest" syndrome. Children grow up, leave home, maybe live out of state.
ix) Sex is marriage is quite variable. Some women regard sex as a chore.
Conversely, some women find themselves in a sexless marriage if, say, their husband suffers from low-T.
My general point is that the lack of emotional and sexual fulfillment is fairly common among heterosexuals, including heterosexual Christians.
That's hardly unique to "gay Christians."
x) Another problem with the "gay Christian marriage" contention is that it treats a homosexual relationship as the functional equivalent of a heterosexual relationship, as if a sodomite who has a boyfriend or lesbian who has a girlfriend is the physical and emotional equivalent of man and wife.
But, of course, that begs the very question at issue. Are these truly parallel? Are men and women interchangeable?
In fact, given the notorious instability of homosexual relationships, it's clear, even on their own terms, that many or most homosexuals don't find homosexual relationships satisfying. Those relationships don't meet their physical and emotional needs. Homosexual relationships are essentially deficient.
xi) Moreover, this really has nothing to do with marriage. If it's just about sex and companionship, that's separable from marriage. Consider all the heterosexual couples who shack up.
Sure, that's immoral–but so is homosexual activity. Homosexual activity is no less immoral if the partners are married to each other.
xii) It's sometimes said that the church needs to develop a ministry to singles, who feel left out or out of place. But since churches are generally comprised of families or young singles who plan to marry, I think older singles will always feel like a fifth wheel. We can have a ministry to elderly widows and widowers, but middle-aged bachelors or spinsters are going to fall through the cracks.
xiii) The upshot is that in a broken world, there's bound to be a lot of headache and heartbreak. That's unavoidable. There are problems in this life that can't be fixed in this life.
Many straight men and women suffer from terrible emotional isolation. Many straight Christians cope with sexual deprivation.
Fact is, life in a fallen world is often hopeless. By that I mean, there's no hope that things will get better for you in this life. Sometimes it will, but there's no guarantee. And sometimes it just gets worse. Historically, that's a very common experience for Jews and Christians. That's very common for Third-World Christians.