This may not be the big blockbuster like Avengers, but they have confirmed a R Rating many months ago, and have a red band trailer because it is most certainly R Rated.
If we didn't get an R-Rated Deadpool there would be rioting.
You know the deal, Fox could no longer control the X-Properties because of repeated Flops. If they continue to make movies like Fantastic Four (2015) that lost money, X Character rights (Including Deadpool) would revert back to Marvel and Marvel Studios. Then they would then be able to introduce the X-Men to the MCU without Singers "worse hack job on a source material story than Michael Bay and Transformers" continuity.
In a way this is the double edged sword, Fox will make a great Deadpool movie, funny, gratuitous nudity, violence, and full of all the adult material we want from the Merc with the Mouth, but that means they get to keep all the X-Men movies coming as well and Marvel Studios/Fox Studios split of the universe will continue.
Why do you think Deadpool was made an Avenger?