They're probably thinking about campaigns for equal marriage as a bourgeois distraction, rather than wanting a denial of rights.
Yeah, they're against marriage in general, so that includes gay marriage.
I personally think that the discrimination of openly gay men (men who marry a woman and cheat on her with men aren't targeted) is caused by patriarchy. Their "crime" is lack of participation in the oppression of women
And I need not explain how the oppression of lesbians is caused by patriarchy.
So, patriarchy is the root cause of it all.
I don't know what this communist party considers the root cause, but they probably think the root cause should be adressed immediately and exclusively, and nothing else should be allowed to distract from that goal.
(I personally think that a real revolution will probably not happen anytime soon, so fighting for small improvements in an overall shitty system is still worth it, but I am old and disillusioned.)