Ok I apologise in advance if this long, as I need to set the scene and also give some background so as not to drip feed.
The background is this, I have 2 dds aged 9 and 6 and a grown up DSD aged 20. My mum and dad are divorced and my mum has been with her current partner for 16 years. In the past I have felt some what uncomfortable about my mum's partner around my dds. The way he sometimes conducts himself around them, sometimes the things he says, my instincts are on red alert the whole time he is around them and I only calm down when he's gone. At first when I started to feel this way a few years ago I honestly thought I was overreacting and being over sensitive. I was possibly sexually abused as a child by my uncle, but can't be completely sure. This has made me very careful who I leave my kids alone with, I've only ever trusted a few people with them (DH obviously, my mum, dad and sister). But DH also has felt the same way, and I've recently found DSD has always felt uncomfortable around him, especially when she was younger.
He's very touchy feely with them, picking them up, massaging their shoulders, 
playing with their hair etc. They don't seem to mind this though, so I feel like I can't say anything when he does it, yet it makes my skin crawl. They both love him, think he's amazing, he's a fountain of knowledge and talks to them both in depth about things they're doing in school. There have been a couple of incidents over the years that have made me very uncomfortable were he has got too touchy feely with them IMO. He also has a tendency to completely ignore when I say they can't do something, and encourage them to do it.
Last year his son moved back home and now lives with them, he's the same age as me. I also get the same kind of feelings of being uncomfortable from him when he's around the kids. Again for the same things, he's very touchy feely, he ignores when I say no, he also gives them both little presents quite a lot and gets into very in depth conversations with them. And again they both love him, so I feel that I can't say or do anything.
As a result I never let my dds sleep over at my mum's (should would love them too, they would love too and I would love them too) apart from the odd occasion when he partner and his son have both been away for the weekend. I also try to always be there to "supervise" whenever my kids are in my mum's. I have never told my mum any of this, because well I have no evidence, and it's not really something you can really say unless you have evidence, I could be being nuts and it would ruin his life. I just tell my mum I find it hard to trust any man that isn't my dad or DH after what happened when I was a kid. She accepts this.
So yesterday we were all in my mum's for Bonfire night. We were having tea and then going to go to the firework display later on. There was a bit of banter going on between DH and my mum (they love winding each other up). Something about my mum saying that women were more mature than men, and DH saying no they weren't and then I said that girls tend to mature faster than boys. Which my mum's partner chipped in with, "Girls don't mature faster than boys intellectually, they do sexually though. I know, I've seen girls of 11, 12 and 13 who are VERY mature sexually." Both DH and I were 
, I was honestly to stunned say anything. It just went straight over my mum's head, as usual.
Can I ask what you would make of that comment?