People are threatened by it. Ive felt that way in the past with other guys and it's probably (surely) due to insecurity in the relationship, in yourself, in your trust of the other person, etc. But really, the only time IMO anyone should worry about their significant other watching porn is if their SO would rather watch porn than having sex with them. THAT'S a red flag.
but honestly ladies, young, healthy men need to ejaculate every day. women dont. we may get horny, and sexually frustrated but we certainly dont get 'blue balls'.