If we want to update hypotheses on multiple pieces of evidence, we need to mentally stay inside the world of each hypothesis, and condition the likelihood of future evidence on the evidence already observed\. Suppose the two are not romantically attracted\. We observe them visit a museum\. Arguendo, we might indeed suppose that this has a probability of, say, 1% \(we don't usually expect strangers to visit museums together\) which might be about 1/12 the probability of making that observation if the two were romantically involved\.
Arguendo: more random non-common latin. Consider "For the sake of argument" or "Perhaps"