The important themes of alienation, grief, abandonment, and even communication could all have been addressed without that much sex.
Another important theme was sex itself. That could not have been addressed without sex.
and she does get some gratification from it (I don't think she would be releasing secretions otherwise),
That is not how the female body works. It does not work like the male body. The vagina actually lubricates itself pretty much at the drop of a hat, even during horrific rapes etc. Best theory right now is that this is a defensive measure, evolved to prevent internal injuries in the event of sexual assault (most animals don't do a lot of asking permission first.)
The woman's motivation is never addressed... but for the life of me, I can't figure out why. In this respect, it's very much like porn.
Motivations can be complicated. She can both enjoy it and loathe it. Again, it speaks a lot to abusive relationships, where there are complex and conflicting desires at work. Porn is sex without any motivations at all.
I find it completely unbelievable that the characters (either of them) couldn't come up with something else to do with their time.
The alien WOULDN'T STOP, so the woman didn't have a lot of choice.
If sex was the alien's attempt at communications, then what kind of moron is it
The whole point was that we and the protagonist couldn't figure out what the alien wanted. Because it was
alien. It might have been trying to communicate. Maybe it thought it was hearing something back, like someone chatting to an AI chatbot, on whatever frequency or method it used. Maybe it was just a mindless creature trying to attack her. Maybe it was insane. It was Other, and they were together; again, a strong thematic connection to an abusive relationship.
The author may have enjoyed themselves, but I did not.
I am getting REALLY tired of people insinuating that Kij Johnson wrote this to get her jollies off. I linked to an interview with her about the piece and its genesis. You can read it for yourself.