I'm not sure if Phil and Sharon are actually divorced but she's living with him, supporting him, running his affairs and being treated as his wife. Non-married partners can be included in wills too.
I thought it was Sharon who suggested he'd cut Ben out because he was gay. Phil denied it and I believed him. It'd be ridiculous anyway since Ben already has a child (as Phil pointed out) and being gay doesn't prevent him having more. Also, why not leave anything to Lexi? The whole will thing was completely nonsensical.
I saw a suggestion on another site that perhaps Ben had been left out of Phil's will in an attempt to push him away so he wouldn't risk his life by donating his liver. I think that'd make some sense, but it doesn't explain the lunacy of the rest of the will...