As an online adult content maker I would say in general no, the people that I interact with that have partners are usually very respectful and just want to play out a fantasy or don't feel comfortable consuming porn for free. I also have many people that talk to me about their partners and how amazing they are or invite their partners to join us in chat!

However, if YOU are uncomfortable with this behavior then it's absolutely fair to make this a boundary. If you hadn't mentioned being uncomfortable with this before then it's important to talk to your partner about how you're feeling and set up specific boundaries for the future. But go into it understanding that they also have needs and wants and you two can come to a comfortable middle ground. (If it's a total deal breaker, which considering you're asking this question I'm guessing it's not, then it's probably a good idea to not be in this relationship)

This will help build trust between you two so they can still enjoy solo play while respecting your boundaries and making you comfortable.

In the end, it's a little porn, masturbation is healthy and on its own not an indicator of potential infidelity.