The husband has already spread the labia and is now using his thumb to press the wide top-side of the shiny turgid helmet of the lover's erection home through the still concealed but probably partly stretched-up opening of his wife's vagina.
When he has achieved that, the firm thinned-out rim of her stretched opening will suddenly slip right over the expanding head of her lover's cock and completely capture it for its first thrust among the lubricious folds of the invitingly clasping sheath of her vagina.
Like you, I find that guiding the head of another man's cock into my wife and watching the rim of her opening slowly yield to it, then, as a gossamer-thin membrane, glide smoothly back over the big firm flange of the cock-head, is very intensely arousing.