Actually I haven't noticed any whims for flirting in my game with specific sims, that could be a glitch being so common or not don't know, I wish we could control it a bit more at least.
https://farm1.staticflickr.com/783/26014275537_e41632977f_b.jpg
https://farm5.staticflickr.com/4795/27016115428_8d3a8ea802_b.jpg
This happened in my game without mccc, now since Don is non-committal and has no friendship or anything with Eliza this didn't actually bother me. See when Bob was working out Don would train him and I didn't realize you built a relationship with your trainer just by training so one day he was slim and they were the best of buds, never flirted before obviously then before he finishes his drink surprise. It would be nice if some sims were restricted like they wouldn't flirt unless you initiated it, like when trying to give the extended family a relationship.
https://farm5.staticflickr.com/4527/38749825681_d6bfc3259c_b.jpg
Like this I didn't even notice until his wife got upset again without mccc though I didn't see who started it so I blamed her sister and cut her out from the family because I love Adam. Lol.