Lucas desperately tried to convince his son not to rescue Mina, but with Dom knowing her location, it changed things, and with more time to consider, Ian knew that he would not be able to live with himself if he actually went through with what he was planning. Even if the baby were not his, losing a child in what could have been a preventable situation was something that he knew would be mentally damaging to his wife. Aside from that, it was highly plausible that she would not want to try again, and left in a situation where he had to consider his wife's emotions before his own, Ian chose to put her first. Love was like that, and while in the beginning he thought Mina losing the baby would fix things, the more he thought about it the more he hated the idea.