unsure why he did this

my OH has a child with his ex. Well she got pregnant less than a year later with the guys she left my OH for. we aren't really close to his ex at all. we tolerate each other and can get along for the kids but that's as far as it goes. any ways we went to my step daughter's activity the other day and ex had her youngest with her. this child is cute but is so mean and horribly behaved. she doesn't listen to her mom and mom doesn't try hard to make her listen. she was running around getting in people's way and ex sat there just saying stop not actually doing anything about it. Well she made a comment to my OH saying doesn't she just make you want to have another one. it's not the first time she has done that but she hasn't done it in along time. my OH always used to never say anything but this time he said no. the thing is is he really does want to have another child with me so bad that he's been asking me when we can start trying and has been asking for over a year now. I'm not sure if he just said no because he doesn't want her to know that he wants another one with me. I honestly don't want her to know anything until after we become pregnant so him saying that didn't really bug me but I'm just wondering why he lied about it to her. i asked him and he didnt really have an answer as to why he did that because he said he didn't even remember saying anything.