In Sir Gawain and the Green Knight, the three kisses from Lady Bertilak are a means for her husband to test Gawain to see if he is, in fact, a true and noble knight. (Some versions of the story include the another angle: that King Arthur's half-sister, Morgan le Fay, put Bertilak up to this, by suggesting he test Gawain.)
The next question is how those kisses affect our young knight.
As a chivalrous knight, Gawain's reaction to the kisses is a serious piece of business. First of all, Gawain is enjoying Bertilak's hospitality: he is an honored guest. In keeping with this, Gawain must not make any inappropriate moves toward Lady Bertilak, but he is also in the unenviable position of not wanting to offend the lady or her husband.
His code of chivalry dictates that he must show respect and concern for women (children, the weak, etc.). He is also supposed to be a man—as a knight—of moral fortitude. Gawain is walking a fine line here. "He...rejects the rules of courtly love by refusing Lady Bertilak's advances..."
...Lady Bertilak is also strongly associated the romantic archetype of "courtly love". As such...the Lady 'becomes the ambivalent mirror in which the knight pictures his own potential for moral achievement or moral failure...'
This means that by making a choice to succumb to Lady Bertilak's attention, or to remain true to his code of honor, he is faced with deciding, clearly to himself and others, what kind of man Gawain really is: can he realize "moral achievement," or will he be stricken down with "moral failure?"
Lady Bertilak should not be dismissed. (First off, if not for her, the test would not be possible.)
Interestingly, the women appear to wield great power. Bertilak's wife is operating unassisted against Gawain in the bedroom as the hunter and aggressor.
Many researchers engage in debates about what kind of figure Gawain is: is he truly one of Arthur's most chivalrous knights, or is his something other: a mix of Arthur's world and the world of magic? My inclination in reading the story indicates to me that he is a man of flesh and blood, tempted by the world like anyone else; he is also trying live up the the chivalric code. To be fair, Gawain is a man first and foremost. It is unrealistic to assume he would not be tempted by Lady Bertilak's acts of seduction. However, Gawain's ability to control himself in the face of Lady Bertilak's aggression is what measures the man: not what he thinks of doing, but the conscious decision he makes to follow his moral compass, courteously refuse, and do so in a way that does not offend the lady, or her husband.
The kisses present Gawain with a moral and ethical choice. And it is a choice. No one makes him select the path he eventually follows. He is a man of conscience. His duty to the Church and his patroness, the Virgin Marry, come first. Then he must serve his chivalric code. Such a life, would in truth, push a man hard in refusing to be moved from his path. In terms of the kisses, he certainly must enjoy them, but he is more dedicated to his code than to pleasures of the flesh, and he is able to resist.