Another thought just occurred to me…I wonder how many people even realize that the meaning of rape has gone through some dramatic changes in recent history. From biblical times up until the modern era, rape referred to the abduction and seduction of a woman still under the authority of her father. In other words, any young, unmarried woman who was seduced OR forcibly violated was raped. In today’s parlance, we might call it statutory rape, but that doesn’t even cover all the possible scenarios meant by rape. A woman under the authority and protection of her father was thought to lack the ability to give free consent to sexual activity. Therefore, it was all rape. Her own culpability only came into question if the rape occurred in a very public location and without threat of violence she decided not to scream for help or fight or run. I think this is mainly because virgins were expected to highly value their chastity and virginity (obviously, right?).
So, even in situations where the girl decided to go along with sexual contact, it was still considered rape, and did injury not only to the girl (her chances at marriage) but also her family (reputation). In those possibly “consensual” scenarios (even though true consent was considered to be lacking), Aquinas would have been quite right to say that contraception would be wrong. Also, remember that the OT (Old Covenant) stated that a rapist may redeem his victim by marrying her!

When I read this as a young girl, I was absolutely horrified. The OT is definitely not age-appropriate reading for young girls, because women were treated as property, and rape was viewed more as a form of theft against the girl’s father than violence against the girl.
Aquinas was likely expounding upon these views espoused in the OT.