It's the lack of control. Vulcan identity is heavily dependent on Vulcans being able to control the extent to which their emotions guide their behavior and decisionmaking. Losing that ability because of biology is going to be stressful, and you probably don't want people you respect seeing you in that condition. I think the sex part is a red herring. Vulcans have sex, they just aren't openly horny except for when their hormones spike and they get too horny to maintain composure. Vulcans know levels of horniness that would kill the average human, were they able to experience them.
Vulcans know levels of horniness that would kill the average human, were they able to experience them.
This is a bit of Trek lore most people don't know about. The reason Vulcans are so big on being logical and having self control is because they're actually MORE emotional than humans, they experience emotions with extreme intensity that often caused them to lash out violently before their cultured embraced this whole philosophy of extreme self restraint.