Hello gocubsgo, welcome to Psych Central.

Emotions are emotions so laughing at an angry person is probably a defence mechanism that naturally comes on to diffuse the situation. We can't really force our emotions to do what we want unless we stuff it away and that can lead to depression. So-called inappropriate emotions can happen anywhere, I've witnessed a person laughing instead of crying at a funeral and I've also seen people cry instead of being happy when receiving a prize. Maybe you could explain to your fiance that you cannot help your reaction as it is an emotion which actually may be driven by fear...