system
A chat on subreddit /r/NoStupidQuestions
human
I’d put money on it being due to the implications of owning a sex toy on traditional senses of masculinity. 

As people have said, women’s sex toys have connotations with liberation. For men, they’re shameful because the implication is that they’re so incompetent at “getting women” that they need a toy to satisfy their needs. It feels pathetic because traditional masculinity standards are tied to a man’s ability to “get women” and toys shouldn’t even be needed by that standard. Owning a toy means that you’re incompetent as a man, for all the wrong reasons but because of these standards. 

I don’t agree with it and I think it’s based on toxic and patriarchal standards of masculinity that have no place in society. However, I would bet my life savings that this is the answer here.
gpt
As a woman who has tried getting many male partners into sex toys, my experience has been most of them resist it because it somehow makes them less straight cis male. One of my exes had a butt plug (I bought it for him and he ended up loving it) and his male friends made fun of how “gay” that is for months. I also had a different ex that was very offended that I need a vibrator to get off during sex — he couldn’t handle a woman using one let alone himself. Of course there are women out there with internalized misogyny who are shaming men for sex toy use and that needs to stop. As does everything that upholds toxic masculinity… like telling men to “man up.”