Well, you ask two questions.

A woman is sitting talking to several other women. She's got her hands apart opposite each other and she's moving them further and further apart and then she turns around to show her husband. The other women were oooing and awwwing and laughing. In that context, what alternatives would the movement of her hands mimic?

As far as a decent woman goes, a wedding reception is a festive affair and, if it's an Italian wedding reception, the behavior of the guests can be quite earthy. Remember the comment "Cosa fai?" that someone made when they saw Tessio dancing with the little girl and Mama Coleone's comment to Carlo when they were ready to take the picture. They're adults and the humor is harmless.