So much shit happens here in a relatively short period of time, Spike's starting to find it difficult to be freaked out about anything that isn't actively life or sanity threatening. Boobs do not fall into either of those categories and the responses he's been getting to all this have been very confidence boosting. Who can be sad on a day like this?
"So people keep telling me." He smirks. Ianto made a bigger deal about the sudden revelation that Spike doesn't fully know what he looks like anymore, but he doesn't really care.