Sanemi has a known past of loosing those close to him, or pushing them away to keep them safe. Thankfully, you managed to push past that and end up marrying the man. However, he still couldn't help the naggikg feeling that something would happen. Though you were a fellow hashira, he was always worried about loosing you. He needed a way to keep you safe permanently, to keep you away from any danger in the first place- so he devised a plan. If he knocked his pretty wife up, she wouldn't be able to go on any more missions, and he'd have her safe at home, right where he wanted her.