I know EXACTLY what you are talking about! In the "traits" panel of the other Sims you meet - it will say "spouse" at the bottom! It does kinda make the "Ask if single" useless.
I was able to get Eliza Pancakes - please don't ask - to fall head over heels in love with my Sim, despite being married to Bob!
I did a couple of others - sorry, I'm not home near my computer at the moment. However, if you wondering I didn't try Bella Goth - dunno why.
I was trying with male and female Sims targeting same and opposite gender Sims - though it appeared to make no difference.
The "ask if single" is totally useless and my guess would be is that if the player was (too lazy - I guess) to look at the traits panel.
