I don't want to answer this question, but I'm going to because I kind of want to cause horrible dreams for you all tonight.
Paul is using the breast pump to develop breast tissue, which will eventually produce milk, instead of using hormones to grow breasts because you know, not actually trans. He is not currently lactating, but soon may, from what I've been able to see from that picture. There is a breast milk/lactation fetish, and that might go into the mommy&daddy/little girl play that terrible trio engages in.
If "mommy" is lactating, Paul is most likely drinking it, at the very least.