WYMM

999 times out of 1,000, when a netizen uses WYMM to mean "will you marry me," it's not meant to be taken seriously. Instead, it's meant as an endorsement of how awesome and/or attractive someone else is.

For example, if a famous zaddy posts a picture of himself looking fly to Instagram, other users might comment "OMG WYMM?" These users likely do not actually want to marry the zaddy (and know that he doesn't want to marry them). They just want to let him know how hot he is, via a facetious marriage proposal.