I'm trying to identify a short SF/Fantasy story of an alien blob lying on a bed in a room where this guy found it and became its lover.
Oh, to Be a Blobel is a Philip K. Dick short story about humans interbreeding with blobs. The humans fought a war with the blobs which involved genetically engineering humans to be able to take the forms of the "Blobels", with the Blobels doing the same with their own people to impersonate humans.
The human and Blobel veterans were then unable to change back permanently, being forced to spend some portion of every day in the alternate form. They resolved the issue by interbreeding.
It could be the story you're after. Even if not, it's a good read, as is the case with most of Dick's work.
Could it be "How's the Night Life on Cissalda?" by Harlan Ellison? It's the story of Enoch Mirren, temponaut, who returns from his trip to an alternate earth:
"When they unscrewed the time capsule, preparatory to helping temponaut Enoch Mirren to disembark, they found him doing a disgusting thing with a disgusting thing."
The story is in the Ellison collection Shatterday