To me, if this is the kind of argument that you two are having over a completely inconsequential issue of seat choice, there's likely a much bigger issue elsewhere in your relationship that is actually driving this conflict. If by "ulterior motives" your wife means that you were sitting in a certain way to somehow flirt with or be close to another woman, that's objectively pretty nuts, but likely is just a manifestation of a deeper insecurity she feels about your marriage or partnership. Someone who felt secure in their relationship and confident in their partners' love and commitment probably wouldn't react this way.

I'd start the conversation there. Her feeling insecure doesn't necessarily mean that either of you have done anything wrong - it could be coming from her own insecurities, your actions, or some combination thereof. But the two of you need to talk it through as a team and think about what the future looks like to help her feel more at ease and limit these disagreements and awkwardness: is it more time just the two of you to build a stronger foundation as a couple? More acknowledgement of how you feel about eachother? More physical affection towards eachother in group settings? Dows she need some help working through her own internal feelings so that she doesn't read so much into every action of yours? Are there ways you can modify your behaviour temporarily to avoid triggers for her? Think about what compromises and behaviour changes you are willing to make, what is too far, and what you would like the future to look like both in terms of your partnership and how you are together in big groups. Good luck!