Androgens are what stimulate the sex organs. The more androgenic steroids like DHT, Proviron or 4AD are a real help.
Deca is not very androgenic, so when it binds to the androgen receptors in your penis they aren't as stimulated as they would be with a strong androgen attached.
I haven't tried deca but I did try nandrosol which is similar. I didn't have any problems that I can remember. In fact things worked fairly well. I did start mixing it 50/50 with androsol since I liked the androgenic effect.
I think as long as you don't chemically castrate yourself with something that has progesterone or estrogen in it you'll be okay.