See, my version of Martin is a bit more devious. If I'm writing this story he's not a bad guy but he's a bit more devious than we know him to be now.
Basically, he started very similarly to your character regarding his girl. Early love and all that but over time he's begun to get a bit bored and he now has his eyes set on Dee.
However, his relationship with his girl is actually stronger and more mature than yours with Dee! They have an understanding (much like in the actual story) that they aren't exclusive so she helps daddy get Dee. There is no jealousy between then and they're more "partners in crime" than outright lovers anymore. Basically, they aren't in the Honeymoon phase anymore. It's purely about sexual desire and Martin wants Dee. All of her. Maybe even more than he let's on to you or even his own girl. Dee is that special to him.
My scenario begins in a similar scene to the BJ scene they already had between you and his girl. Dee only watched then but this time you can choose to let Dee blow him.
If you do, you're making a choice about PUSSY. It's the player (you) saying I'm playing for pussy so this part can be explored.
Anyway, during this BJ Martin can't take it! Dee is just too sweet and still so new and fresh. He has to have her. So, we show the men look at each other and show equal satisfsction. That's when Martin gestures to you to "take her upstairs" , meaning his girl to do as you wish.
If you allow this you leave Dee alone and before she knows you're gone upstairs, she's alone with Martin, still blowing him.
That's when Martin makes his move. Dee is already laying on her belly, on the couch blowing him. Martin starts by rubbing her ass. Dee says nothing. Martin then rubs her pussy under the skirt, over the panties. Dee looks at Martin but he tells her "it's ok" and she continues sucking.
Dee stops in shock when Martin slowly tugs her panties down. Dee only now realizes that you're gone! Shes alone with Mattin! She's confused but Martin again reassures her, now kissing Dee as he repositions himself over her.
That's when Dee first hears Daddy upstairs fucking Martin's girl. He hears the moans and bed squeaking. She's unable to say anything in her shock and confusion! She's jealous but also into it, wanting to stop Martin but also kinda into it. She's so unsure...
Martin isn't. He needs to be inside this angel and before Dee can say a word he's pressing his dick against her pussy.
"Wait" Dee whispers, just as Martin's fat dick splits her pussy lips. He moans in pleasure, she let's out a slight whimper in pain and uncertainty.
Martin gets his prize but he's not done. This is the beginning...
Upstairs you're fucking his girl in ways she's never even imagined. Yet, you only now hear Dee's whimpers of pleasure and begin to both second guess your decision and Martin's motives. It wasnt supposed to go this far so fast.
End scene. That's how I build Martin from here. He's NOT the guy you thought he was. Not evil but he definitely has motives. Also, maybe this moment pushes Dee away a bit and she begins to experiment on her own? This would open the options mentioned above.
But this is all just me...