I think he likes getting attention from women. It might be perfectly harmless, but he might be insecure and/or an attention whore. If you have no reason to suspect him of cheating or doing anything wrong, then don't assume that he is.
as for why he doesn't have the husband's phone number in his phone, there could be lots of reasons for that. 1) The husband may not like doing a lot of texting, etc. on his phone. 2) The wife might be the main communicator for both of them (especially if the husband is a little more introverted). or 3) Your boyfriend may just be more comfortable talking with women (which could be because of his attention-seeking tendancies).
Deal with the two of you based on the two of you. If you have concerns, ask your boyfriend, but not in an accusatory way. Just talk to him.
Good luck
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