Well back on topic again.
Is the way Shirou reacts really that weird?
Shirou probably reacts the way he does because of his morals and ethics.
You know even though many people accuse Shirou of being sexist and sometimes just plain dumb I would actually say that he is in many ways quite the gentleman.
And just so that people don't get confused about what kind of gentleman I mean.
"In another sense, being a gentleman means treating others, especially women, in a respectful manner, and not taking advantage or pushing others into doing things they choose not to do. The exception, of course, is to push one into something they need to do for their own good, as in a visit to the hospital, or pursuing a dream one has suppressed." http://en.wikipedia.org/wiki/Gentleman
And I really do think that one should also consider the fact that they are in life threatening situations in both Fate and UBW.
In Fate they are being chased by Berserker and all of a sudden Rin comes up with the splendid idea "lets have sex! that will save us" which seriously should kind of mind fuck Shirou.
And about the "UBW fun scene" I must say that I kind of disagree with Swartt. I am actually quite sure that Shirou is somewhat aware of Rins feelings towards him at the time of "sexual intercourse" however at the time they are in a hurry to come up with a plan to be able to fight Gilgamesh and not getting killed so that they can stop him from using the holy grail. So in UBW it is really something like this.
Shirou: Ok Gilgamesh already has the grail. Archer could probably defeat him so theoretically I should also be able to defeat him however I know that I am nowhere near as good as Archer in terms of abilities however it is still worth a try since I know that I cannot really just give up and not do anything about it. However just in case, Rin might have come up with some kind of solution so lets ask her.
Rin did you happen to come up with a way to defeat Gilgamesh?
Rin: Yeah in a matter of fact I did.
Shirou: Oh that?s great so what should we do?
Rin: Lets have sex so that you can gain super powers and become able to fight a bit more like Archer and btw become able to use your reality marble.
Shirou: Seriously?? Sex?? Could that really help??@_@
Well perhaps a bit exaggerated however that is pretty much what happened in Fate and UBW I guess.
And I also think that one should consider the fact that he has not known the girls more than about a week or so. And I don't really think it is that weird to think that you are not ready to have sex after such a short amount of time even if you have gone a date or not. And to have sex with someone under a stressed life threatening situation should somewhat go against the "gentleman lifestyle" that Shirou seems to somewhat follow.
To be honest I think that I would have acted almost exactly the same under those circumstances.
And regarding the Fate epilogue I don't really see why he should grief. Saber devoted her whole life to her country and did many great things but got betrayed in the end however that does not change the fact that her acts were still good ones.
And I also think that she somewhat accepted Shirous "You cannot change the past" philosophy. So in the end I think that she accepted her life as an extraordinary and good one and that she had nothing to regret. And I also think that is the reason for why she did not choose to stay. After all, she did not even belong to that time period to begin with.
She lived her life as she thought was best, she did many great deeds and regretted nothing and in the end she even got the added bonus to be able to experience true love. So should Shirou really become all emo and so on just because his love got to experience all that she wanted plus more?? And if I remember correctly Shirou said something like "I do not want to sully her pride by making her stay" or something. I am sure that he thinks it is sad that he could not spend more time with her however she had no regrets and died happy so he really has no reason to grief or become emo or anything like that.
So this is my thoughts on these matters. I do not know how many others agree with me however this is how I interpreted these things. And btw should I bother to use spoiler tags even though the topic says "spoilers"?