in shoujo manga why does the boy put his hand into the girls mouth when having sex?

[NarutoeGAJM]

when in a dominant shoujo manga story when they are having sex the boy is sometimes usually putting two or three fingers into the girl mouth in the occurrence of this. why is that?
if you want to see by what i mean go here

http://mangafox.me/manga/rhapsody_in_heaven/v03/c013/33.html

be aware though, this is a sexual reference! so shoujo manga lovers only!