I'm going to encourage my girlfriend to lose weight
Are you wanting to encourage your girlfriend to lose weight so that you will be happy?
Yes, it would make me feel good.
It is more important that she wants to lose weight for herself rather than you for her. It's good you care about her weight, but make sure she is ok with this and can commit to it.
I'd be more inclined to be intimate with her if she lost the weight.
It's great that you want to encourage your girlfriend to be healthier. Losing weight may help your relationship and it will improve insecurities that she may have.