He better be. The trio of aspects are VERY devoted to all of Shiva's aspects. There's a reason all 3 are 'Sakurafaces'. Cheating isn't likely to happen. You're more likely to cause Kali to manifest. In a blood rage. Errr...beyond her normal one.
and? Archer is still Shirou, it's just as a servant he should be able to survive three very horny sakuras.
He really isn't that much Shirou anymore. And considering what the HF material book has mentioned about how Archer doesn't care much about Sakura, and how no Sakuraface has any real reaction to Archer (beyond Meltyiris), it makes me roll my eyes at stuff like this honestly.
It would've fit better if the hand and shadow had been Shirou, not Archer.