I have to admit, I also get very aroused hearing/seeing my wife have sex with her boyfriend..Very much so. And she knows I do (and he does also) and we are very open with each other about that.
It's also a good tool to use if I ever get that occasional tinge of jealousy, I just remind myself the pleasure I get from it as well, and needless to say that jealousy disperses pretty quick. It's just another form of compersion for me.
Is it normal? I guess it depends on what each person finds normal. I find it normal, but that's me. Others may think its strange but that's their opinion, which they are free to have.
The characters in another chapter we call life!
T - Wife - hinge in Poly V
J - Husband Poly
F - T's Boyfriend Poly (good friend of J's)