Christina Aguilera is one hot mama!  The pop star and fiance Matt Rutler stunned at the Apollo for a fundraiser in East Hampton, New York on Saturday.   Xtina looked gorgeous in a plunging black sequined dress with her hair styled in tousled curls, while Matt looked dapper in a gray blazer, a black button down and black skinny jeans.  Later in the evening, the mother-of-two took the stage wearing a second cleavage-baring gown, and dedicated a song to daughter Summer Rain's papa.  The cute couple also celebrated their adorable baby girl's first birthday on Sunday. The 34-year-old likewise took to Instagram to mark the special day with a super sweet shot of her little girl.  "Happy 1st Birthday! My Sweet, Summer Rain," the "Beautiful" singer posted with the cute candid.  Aguilera and Rutler -- who have been together since 2010 -- welcomed little Summer on August 16, 2014. Christina also has a 7-year-old son,...