Alma, I've been eight times in SF and there is always laughter at this moment, but I don't think it's malicious. Here's my take, I posted this a little while ago:
I think the laughter is two-fold. One is surprise, even Jack and Ennis (especially Jack) is surprised that this guy who left him last time saying: see you around, is suddenly devouring him with total abandon. I think both of them are blindsided by the depth of their need and passion for each other in this scene. Even with all the anticipation (Ennis nervously waiting, Jack red-lining it all the way there) I think the sudden torrid, unstopable need in both of them is a bit of a surprise, the undeniable ferocity of it. So, there's that: the feeling as a viewer of HOLY HOLY SHIT, the giant WOW. But I think it's also relief both for the audience and the lovers, so much pent-up desire, so much dissatisfaction in their lives, marriages, so much flatness and gray, cramped quarters, compromises and then this explosion of emotion, passion, love that is pure, real irrefutable. So, I think the laughter is also from relief. AND in relationship to Alma, I think it's not for lack of sympathy with her but frankly, I think in this scene you want e & j together so badly, sympathies are more with the lovers, they have to be. There is also an element of uncomfortable laughter at her shock, a "girlfriend you have no idea. . ." type of response. But I think, at the same time there might be this laughter, it is not directed at Alma or out of a lack of empathy for her, just a wooooaaaaaaaaaaa reaction to the situation. . .my take on it, anyway.