@Iwill , just to expand a little on @teacake 's answer…
Google and other search engines don’t allow female nipples without labeling the sites, “adult”. This double standard, rightly, annoyed Brenna so the rule was expanded here to ban all nipples as a protest against inequality. There is quite a lengthy discussion of it in the Emily Clarkson campaigns to ‘free the nipple’. Sponsored by Lovehoney. Hypocritical? thread.
Please, any who haven’t seen it, have a read, it goes into full details of why male nipples are also banned on this forum. Essentially though, it’s a statement against gender bias. If the internet rules change and female nipples are no longer classed as pornagraphic by search engines, hopefully all nipples will be free to be seen on the forum if their owners so desire.
ETA I’ve just reread the thread I linked, it seems some of the more “heated debate” has been removed 
. The general discussion and reasons are still there, though.