on the face of it, sounds very cruel and manipulative - however without context (has OP been constantly harping on about marriage etc), it's not clear cut.
Scenario 1:
First time question OP > DP: Would you like to get married?
First time response DP > OP: Fuck off fatty, lose some weight.
Scenario 2:
OP>DP - every week - marriage, marriage, marriage - friends just got married was nice, ooh look rings, ooh look dresses, oooh look venues
DP>OP - feigns interest initially, makes jokey comment (that turned out badly) to downplay it.
Not saying LTB as no-one here knows the ins and outs of the relationship - on the face of it this does seem cruel....... but, y'know - Size 18 is on the large side, so losing some would be healthier, even if you do decide to stay with / dump this chap.
As for the poster above who said "my friend lost the weight for the wedding then put it back on again once married" = grounds for divorce, simple as. No excuse for giving up when you get married, kids or not - it's lazy and unattractive, and easily avoidable.