The difference is the PLP shippers are genuine but the HLP shippers joke. We have no sympathy for genuine shippers but comic shipping is hilarious.
To clarify the difference between the two tropes further, one (currently called PLP) should be for sexually compatible couples. Straight man and woman, two gay women, whatever. But the compatibility is the key. One of the redirects is therefore "The Straight Will And Grace
". Will and Grace themselves should not
be an example, nor should any other gay-man/straight-woman pair.
The other (currently called HLP) should be for sexually incompatible couples. If they are two gay men, then they are not this trope, even if they are men who act as close as a couple without being one.
These tropes aren't really about heteronormalcy. It's about the expectation (reinforced through years of TV) that if two close characters can
get together romantically, they will. If two people are sexually compatible, do not connect romantically but still bond as closely as a couple, this is notable. If two people are sexually incompatible but bond as closely as a couple, that too is notable, but for the opposite reason.