Welcome to JustAnswer.
You said that your husband tells you when other women "make sexual innuendos toward him". What else has he told you?
From the incident you describe she was doing nothing toward him - she just sat down. And I don't think he did anything wrong - he did notice that a woman's rear was hanging out - but he would have noticed that no matter who that was, right!?
Secrets are the thing that are dangerous - not what he's telling you. I think this is just your insecurity - maybe this is the way you usually are - or maybe it's the pregnancy (twins do tend to cause more whacky hormonal things - I hate to generalize - but it could be...?)
Unless something really stands out with what else he tells you... this one incident doesn't sounds like anything to be concerned with - just something odd that happened - and he did get up and leave - not sit there enjoying the view!