How could anybody be okay with having the man they were in love with go off with someone else?

Spock and Kirk's relationship with each other is clearly the primary relationship in each man's life. In large part, the vagaries of 1960s television writing are the cause of that: there was no such thing as a story arc, so a spouse would either had to have been introduced as a character from the outset, or it would have been established that one of them was, for instance, a widower. But for whatever reason, the choice was made not to really address spouse/marriage issues for these characters who were in their mid-thirties. (Okay Spock had a failed marriage, basically; and Kirk had no time to find a woman, nor did he make much effort, and in fact he stopped complaining about it more or less after Naked Time.) And then the times they began to change and Stonewall happened and over the years it became more thinkable that if you analyze their situation with Ockham's razor, they were deriving their primary emotional support from each other.
Now I totally respect the opinion of people who say that may be the case but you can't assume a deep emotional relationship always leads ot sex. It does not, indeed. And if a person doesn't want to see it that way, there is no reason why they should have to. That, actually, is one of the clevernesses of 1960s television writing: like a set composed of abstract plywood shapes and colored lights, the storytelling left plenty of interstitial spaces that you could fill in your own creativity. (I'm so going to reuse this line in a later post.)
In the interstitial spaces I fill in, there is no reason why they wouldn't consummate their relationship. Although I have to ask myself if Kirk's two or three legitimate girlfriends during the five-year mission were not a little case of this:

I remember an older acquaintance, "Joe," who lived in Boston and had a lover we'll call Frank. … The two lived together, travelled together, the two seemed in every way a couple. …

On one trip to Boston I ran into Joe who told me that Frank had married a woman. He said he knew he liked women and was perfectly okay with it. How could anybody be okay with having the man they were in love with go off with someone else? I never did get all the facts. Was Frank bisexual, and did he prefer this woman (or perhaps more to the point, a straight life) to living with Joe? Had he only been using Joe (I mean, who was paying for their vacations together?) While I seriously doubt if Frank was totally straight, perhaps he -- and even Joe -- thought of himself as a heterosexual. Was Joe so in love, so lonely, that he'd take up with a "straight" guy, knowing all the while that he'd inevitably walk out on him? [Was Spock?] Was Joe invited to the wedding? Did he stand there pretending to be happy for the man he loved while his heart was breaking? Did he make any attempt to make Frank see that his marriage could merely have been an act of internalized homophobia? (Joe was not exactly an activist type, however.) We lost touch and I never got the answers to these questions, or learned how he ultimately dealt with losing his companion. I don't even know if he's alive.

Bill Samuels | J.A.T.G.A.B.
The progression of Jim and Spock's relationship begins to take shape: closeness forms after Gary Mitchell's death; they begin to see how much they mean to each other during and after Naked Time, but Jim continues to hurt Spock with these occasional girlfriends. While at first Spock felt he had no right assert a claim to Jim since he himself was claimed by T'Pring, after Amok Time, Spock begins to want Jim for himself but Jim still has these illusions that a woman somewhere is going to make a better partner for him, and eventually Spock is hurt enough that he goes to Gol, but they mature and are reunited.
But yeah, they do undeniably not act like out and proud gays. Now that may be because the 23rd century is post-gay as in there is not a sharp line between gay and straight and it's sort of irrelevant, but for purposes of thinking with the myth in present day it is problematic. Is this a myth that suits gay men in the real world? Or is it a myth that was suitable for the 60s through the end of the century but has been outstripped by reality?
As a child of the 60s I will always love it as an adventure and as a love story, but if it offers no suitable mythology for current times, then... we pack it up in tissue and smile when we remember it.