The Bellas actually won the Riff off...
The lyric of "I'll Make Love to You" that the Bellas were singing was "...and I'll hold you tight."
Which means that if Jesse was coming in on the "I" in "I'll" to sing "and 'I' guess that's just the woman in you that brings out the man in me..." then the Bellas got cheated. However, if he came in on the word "and," then they won fairly because the rules that you can't add suffixes weren't adhered to.
However, when Becca starts No Diggety, the Bellas win because it's is the contraction of it is, Becca actually came in the way she was supposed to when she came in on the "It" of "it feels like the first time and just added the 's.