I noticed this EXACT thing when I had started dating my now ex-girlfriend.

So. Much. Attention. That it led to silent resentment against her almost!

I thought about it and chalked it upto 2 things:

1. "Pre-selection" - women tend to find men who're "pre-selected" or "vetted" more attractive. When I was out and intimate with an attractive girl, naturally other women found this a "pre-selected" safe bet.

2. No neediness - no matter how much "abundance" in female options a man has, some level of neediness exists when he's single. But in a committed relationship? This vanishes. And this consequently is bound to increase his attractiveness.

I also think my ex could bring out the best in me - otherwise, I come off as a "cold arrogant stoic" dude. With her, my playful laughing warm self would come out...

Maybe this could have been a factor too.