<human>: My significant other and I have been friends with another guy ever since college. In the last year, he has picked up a nice girlfriend who we all like. She happens to have a very large bust and has no problem wearing clothing to show it off. Her and my SO have no problem basically joking about it in party environments, and my SO will jokingly do things like lay her head on this girls bust, etc.

What I'm not sure how to react to is both of them jokingly inviting me to look at her chest. This isn't something that is constantly happening, but seems to happen at least once for every party we're at. It usually goes along the lines of my SO laying her face there or motioning her hand under her bust and saying "Hey...HEY come on take a look! Come on! You know you want to!" Often the friend joins in as well. This is purely just a friendly teasing thing, nothing really sexual about it, but I'm not sure of the proper way to act.

As of right now I just do the whole "Nope, nope not looking, just staring at my SOs face and not the boobs" thing, to which they are always saying jokingly "Wow, look at that will power." The issue is, once it starts, this may continue for 10 minutes where they keep teasing about it. It just starts to get irritating after a while. I don't want to be a downer and tell them to knock it off, but at the same time it almost feels like I'm NOT being masculine if I refuse to look. So, I'm curious of input on this.

Give me the gist of the text in a nutshell
<bot>: So keeps jokingly asking me to look at female friend's chest, I have no idea what the proper reaction is.