Nan’s vessel comment

Will you help me unpack this please? So, let’s accept that Nan was “forced” by the Dowager Duchess to marry Theo (I posted how I’ve been struggling with this but okay, let’s accept it). Nan has been having sex with Theo for months, which she must have known could result in a child. In Episode 3, she still accepted Theo’s lovemaking, before he stopped and told her they could separate after an heir. That she protested against, she did not want to be used as a “vessel”. Which is fair, but I wonder —-

So the issue was with Theo’s intent? Nan entered the marriage transactionally and had sex with Theo. Now that Theo was aware of the transactional nature of their marriage, them having sex for a child was suddenly bad? So when it was transactional from Nan’s POV, she was not just a “vessel”, but when it was transactional from both of their POVs, she was? Yes, she reached a point where she wanted to make their marriage work but before that, she was already having sex with him, which could have easily resulted in a child.