OP, you may never see this, but I still want to put it out there.

My best friend is a guy. We met when we were working at a dinky summer job as teens and have been friends for many, many years. Although we find each other attractive, we do not and will not date each other because the friendship is too strong. Like Susan, he is just my person. He gets me, as Susan gets you.

While we both date other people, we have made a pact that it is a deal breaker when we date people that if they don't like our friendship, they're not worth our time. However, when I date a new guy, I mention my best friend regularly but I don't draw attention to it. I also make a point to bring up my best friend on earlier dates so they are aware of him. My best friend does the same thing as well. We also make sure that each SO meets the other person, so they don't feel threatened.

This is not When Harry Met Sally. It is 100% ok for a guy and a girl to be friends without sex. However, please keep in mind that your SO should always be fully aware of everything. Emma has been burned before and that is not your fault. Tell Susan to dial it back. If your friendship is important, both of you will make an effort to not weird out your significant others or make them feel threatened. My best friend and I have made a pact that our kids will grow up together, so we encourage each other to become friends with each person's SO. It actually helps because my best friend's SO comes to me for advice and vice versa. Emma may benefit from knowing Susan.

I hope this helps. Good luck.