I agree with most of what has been posted. I also think you should try to forget you ever heard this saying as by buying into it even a tiny bit, you run the risk of it influencing your impact on your son's future relationships.
I strongly suspect my MIL is a firm believer in this saying. As a result, she treated me like shit from the first day that I met her, and when my DH told her he was going to propose (after 5 years together), she tried to talk him out of it - while I was sat in the same room. I can honestly, hand-on-heart, say that I did nothing to deserve her hostility; I tried so hard with her for a good few years, and badly wanted her to like me or at least tolerate me.
Funnily enough, once we had our first child, she started to make a real effort with me - almost like she suddenly realised that I wasn't going anywhere and her hostility towards the mother of her sons child could lead to her potentially losing him and her grandchildren. By this point it was too late for me, and while we have a good relationship on the surface, I really don't like her and it's all for show and to make DH's life easier.
DH obviously loves his Mum but years of her hostility towards me, PA behavior and guilt tripping have led to him making less effort and it's actually me now that reminds him weekly to contact her/organize visits etc
I think if you have a good relationship with your son and don't interfere with his relationships there is no reason at all why you would "lose him" to a wife. Making an effort to welcome his future girlfriends into your family would also help, I'm sure.