"Part time wife" sounds like an attachment issue, or if you don't like attachment theory (which is fine) entanglement and dependence. I mean, you go out with friends right?
I totally get what he means by "part time wife."
Sure, mono people have their own lives, go out with friends, have hobbies or may occasionally take separate trips, or have intense work schedules. But my poly exbf felt unavailable to me when he was with his secondary in a way he simply wasn't when he was doing anything else. It was a part of his life where I was not included, wanted or welcome, it was the one time I *had* to keep away. It was not at all like if he was just with friends or family or enjoying a hobby.
Plus, poly cuts right into relationship energy. For example, I personally prefer to spend every night possible with my partner (barring rare occasions,) not just 4 or 5 nights a week so they can be with someone else on the other nights. For me, that feels like half a r'ship. Romantic Getaways? You only have so much money and vacation time, so who you gonna take on that romantic getaway? Me every time, or me half the time?
It doesn't mean the guy is codependent if he doesn't want to share his partner with another partner. It does probably mean he's monogamous and won't feel satisfied in a poly r'ship.