I like that you've led with your intention (that you don't want to sound presumptuous) and then you check in with your own needs (my own comfort level) as well as their autonomy (free to see other people) and then you have your question (are they seeing other people?)
Do you also have a question that would cover future desires for other partners/changes in fluid bonding? ie - they're not seeing anyone else, and so you feel safe fluid bonding with them, but that you'd want the courtesy of a heads up if they were going to be involved with someone else down the line so you could introduce protection again?
In some ways, I feel like you feel almost apologetic for your needs around safety, and desiring exclusivity to feel comfortable fluid bonding. I think that it's very important for you to know that these needs/desires are very valid and worthwhile, and that you don't need to feel like you have to be so careful when it comes to their feelings when it is your health and safety that you are concerned about. Fluid bonding ups the intimacy in a relationship, brings up STI and pregnancy risks and is another step in equalizing sex that you share with your partner. Take these steps slowly, at your own pace, and on your own terms.
A large part of great sex is feeling safe, respected, and comfortable being yourself. I think this is a great opportunity to create open communication about your needs, wants and desires.
In context - I am a lot less subtle when it comes to my own safety. We were in a quad last year, and fluid bonding came up as an option. We were all tested, and had both stated that we were not seeking out other partners and were exclusive to our quad. My partner has a vasectomy, so I'm not on any form of birth control. I was using condoms with my new male partner as the risk of pregnancy was still a factor even though STI risks had been negated. I let him know that I was unwilling to go on the pill - he suggested the morning after pill and I literally laughed at him and said that I loved my body way too much to put myself through that for his pleasure - he ended up getting a vasectomy because I wouldn't budge. His wife was THRILLED as she's been trying to get him to have one for over a decade. Ended up being a happy ending for everyone LOL. He sent me paperwork to prove that he had the operation, and that he was officially shooting blanks, and I sent him a "Enjoy Your Vasectomy" e-card that had him howling with laughter. Thought I'd share my experience with you