A: Aroused
Nothing turns you on more than knowing that your hothusband/hotwife is getting all the sexual satisfaction that they deserve and that you were never able to give them. To know that another man is giving them everything they always fantasised about is exactly what makes you leak precum -- and presumably strain in your little cage.
This is a new series of short text-only post I'll be working on. Feel free to send me suggestions, especially for some of the less obvious letters.