I think the booing was for several reasons:
1. Melendez is the hometown fighter.
2. Many people there probably biasedly thought Melendez won.
3. Other people probably unbiasedly thought Melendez won.
4. After all of the explosive finishes, many were probably impatient that the main event didn't finish the same way. I mentioned before MMA fans can be fickle. I've been to UFC in San Jose before and believe me when I tell you people were booing Shogun vs Hendo in the audience before that fight was over.
5. Bendo might be a nice dude really but he isn't beloved or anything my most.
6. Wedding proposals as PDA's are not entertaining to anyone unless you care about those people on a personal level. It was a little tacky.
7. Many were booing because they thought that poor girl deserved to receive a marriage proposal over a nice romantic t-bone steak dinner and not in a blood and sweat soaked combat arena.
8. YO! DAT DUDE DIDN'T GIVE GILBERTO PROPS!!!!
9. Bendo might of lost a fan or two with the late break at the end of round 3 (yeah, I'm running out of reasons).
10. Last fight of the night, everyone is liquored up!!!
That said I cringed but did not boo. I vote disrespectful because tha's how I roll. I get down right angry when idiots boo good MMA. I didn't like the post fight booing during the proposal but given the cards were literally just read seconds before, Bendo could not of picked a worse time I think.