I am wondering if you would prefer/feel comfortable if she fall in love with somebody because that feels safer to you. If you think about her engaging in casual sex instead, does that make you fearful or like what you share with her would be less special or diminished?
I do see a lot of people tend to be more OK with the sex and less with the love, but I have seen people post about feeling the opposite, and those are some of the reasons. Especially if there wasn't a history of lots of sex partners before they got involved with them. Especially if they thought their partner never lusted after others until the talk about poly came up. Especially if they thought they were only interested in dating because they want to explore bisexuality and then one day are told that they also are interested in heterosexual sex with others too.
They see their partner is a certain way, and realizing that their partner actually enjoys sex without romantic entanglements can be eye opening in a way they don't like. At the far end of it, they can feel like this discovery means their partner is a "slut" and they start having negative feelings about them, and any relationships their partner forms that don't fit the "idealized" format.
Hopefully that's not the case, but if it is something that might be possible, you might want to be aware of it. Maybe your wife only wants serious romantic relationships, but once you change the dynamic of a relationships there are lots of unforeseen possibilities that could happen.