(this may be my inner fanboy talking)
and with that said allow me to set the stage first:
in the middle of the epic final battle in Episode 3 with advisors, combine, zombies, and resistence fighters all present Gordon and Alyx are back to back in the thick of it all fighting for thier lives and watching friends and enemies alike die. Gordon suddenly stops turns Alyx to face him and kisses her for all he's worth and suddenly everyone stops the killing and watches.
Alyx: Gordon.......wow this was unexpected
Gordon: I've wanted to do that for the longest time now, you know for if we don't make it
Another kiss between the two and then Gordon launches into a group of advisors and the chaos begins anew