In his genetic code?? Is he right?

Hi ladies. This is my first time posting here but I really need some advice, I’m not sure if I’m overreacting or if I have a right to be kind of upset about all this.

Ok so, my SO and I work together, we met at work and have been dating for about 2 and a half years now. Hes never given me any reason to doubt him or worry about him cheating or anything like that but I do have insecurity issues from a past relationship (issues that hes aware of and that we have discussed and I am working on as best I can) ANYWAYS theres this woman that we work with that started working there almost a year ago now and her and my SO are both managers at work so I understand that they are going to talk and its part of his job to talk to the other managers etc. but the issue I’m having is that I am kind of friends with her, we text and talk a lot at work BUT I have caught him checking her out several times (staring at her butt or at her chest) and its making me feel like shit because shes in better shape than I am and alwaysssss has her hair and makeup done up at work rlly nice and overall I just feel like shes prettier than me. And everytime I’ve caught him staring at her he denies it and makes me feel like I’m seeing things and like I understand that guys sometimes check out other women but do you have to do it right in front of me?? And to my friend??

Its frustrating me a lot and its been causing arguments between us and he just keeps telling me that I just need to “get over” this issue that I have. But yesterday we talked about it and he admitted that he does check her out and said that “its in mens genetic codes” to do that and that its “really hard to fight that urge” and I don’t know how to feel about all of this. I’m upset because this has been happening for months and hes been telling me that I’m just seeing things but now hes telling me that it IS happening.

But I’m really frustrated with myself for even being upset about it because I know that hes allowed to find other women attractive :( but him saying that its part of his genetic code is like basically saying “get used to it”. Right?? Am I overreacting? Or should I be able to expect him not to check out women at work in front of me, especially my friends?

Sorry for the length I just really need to vent about it and any advice would be appreciated