There are some things you should simply should not do without asking, and even with permission, they sit on the line of selfishness and bad form. Proposing at someone else's wedding is absolutely one of those things, even if the couple gives you permission, it looks tacky and makes their special day about you.
On the contrary, it's completely natural to feel extra romantic if you're attending a wedding with a long-term partner. Watching people declare their endless love publicly is bound to trigger some reflections about whether you're looking to tie the knot or push your relationship to the next level.
Still, feeling touched and inspired by the romance of a wedding is very different than deciding to get down on one knee at their reception, effectively making their day about you.
As you can guess, the internet gave him a deep read.