Aquinas's understanding of conception and how women fit into the whole picture was completely wrong.  He believed the entire person was in the sperm and women were just a sort of plowed field that sperm planted in.  His entire theology in this area was totally male centric and it's one of the reasons male masturbation became such a mortal sin and by extension any kind of contraception that interfered with the proper placement of sperm and it's ability to 'seed' itself.  In a real sense, Aquinas saw masturbation as a sort of male abortive act.  That initial thinking dominated the moral reasoning all the way through Church teaching right through to HV.  Even though HV tried to incorporate the idea of sex as a marital bonding tool, that reasoning never surpassed the 'sperm is sacred' reasoning.