The more obvious problems have been discussed elsewhere, such as the violation of the 7th commandment and the violation of the “male/female He made them” natural law.
I don’t think I’ve seen the 8th commandment addressed though. Still, once you think about it, it’s obvious.
When you pair off in what amounts to a marriage relationship without sex, you steal each others’ ability to seek marriage, have children, etc. This is why it is extremely important to debunk the idea that homosexuals cannot ever marry the opposite sex. We know they can because we’ve seen it. We know they can because the men often look at heterosexual porn. We know they can because God is able to change people. We know they can because “such were some of you.”
If it were true that they could not, then the justification of claiming these so-called spiritual friendships add something that is missing in their life and that they cannot get any other way might make sense. But what is actually happening is they are taking away the last vestiges of a chance the other has to get those good things in the only way they can truly be gotten. When you make a covenant with each other in that way, it is covenanting to do harm to one another. To steal from one another.
I know of at least one homosexual couple where this is exactly what happened. Living together for years without sexual relations, one had her chance at marriage and children taken from her and only realized it too late.
I also know of a heterosexual couple where this happened.