Nervously, you approach Sam after graduation, telling he you want to talk in private. Out back in the woods behind the school, you ask him to marry you, as your wife. You explain your father's backwards views and the business you're set to inherit, how much you love Sam, and that if he truly wants to remain a man you'll find a way to make that work instead.
"Actually Ty, I'm so happy you asked", Sam replies. "I've been fighting the obvious for too long. Ever since I hit puberty I've felt like I was meant to be a girl. I've been trying to fight it, with the short hair and my obsession with running, but I can't fight who I'm meant to be. Besides, look at how girly I already am? I'm gonna be super sexy as a woman."
"Wow, I never knew..." you reply.
"There's just one thing though..." Sam says, trailing off.
"What is it?" you say.
"I want to have sex using my dick before I become a woman. Just once. It'd be a shame to never get to use it."
You ponder his request for a moment, but eventually agree to his condition. "Tomorrow night we can take my car down to the old reservoir.
I've got some condoms in my bag we can use so I don't...you know..."
"Yeah..." Sam replies. "Alright, tomorrow night! One last hurrah as a man and then on Sunday we tell everyone the good news!"
The next night, you're both sitting in the back of your car at the reservoir, seeing each other naked for the first time. Well, seeing each other's dicks for the first time. You've been topless around each other before. Made out a few times, groped each others' breasts, the kind of thing teenage lovers get up to.
Both of your dicks were the same size. Not surprising, there wasn't much variation in size amongst males. The rest of Sam's body totally outclassed yours though, in both female *and* male proportions. Sam was toned, with visible abs and even noticeable pectoral muscles in spite of his impressive breasts. You wonder for a moment how big his are going to get. When male humans become female their breasts typically grow about 3 cup sizes, which in Sam's case would make him an E cup. But people had been know to grow as by as little as 1 or as many as 6. Meaning Sam would at least have large breasts, and might end up with truly massive ones.
Those thoughts could wait for later though. Sam had gotten the condom on and was absent mindedly groping your left breast. You lean in and hug him close, giving him a deep kiss and feeling the stiffness of his member against yours. Feeling more than a little nervous, you attempt to lean back slowly but end up flopping onto the bench seat with a thud. Sam slides on top of you, lines up his dick and...he's in.
You feel...weird. As Sam slides his dick in and out of your rapidly moistening vagina, you don't feel any pain, but you don't feel any pleasure either. You just feel uncomfortable, a tingling sensation in your lower crotch as if your leg had fallen asleep. Above that feeling, however, was a familiar feeling of pleasure. Your dick aches, throbbing as he thrusts. Though neither of you is touching it, it feels better than it ever felt when you masturbated in the past.
After 5 or 6 minutes, you hit your limit. "Sam I'm coming!" you say, putting a hand on your dick to direct it to the side and not spray him. "Me too!" he replies, stopping his thrusts and letting out an exasperated grunt. You feel two or three strong twitches from inside you as Sam fills the condom, and then the two of you lay still for a minute, recovering. Sam pulls out, removes the condom and slips it into a zip lock bag to dispose of when you get home. You take a rag and wipe your own cum off your leg. The two of you get dressed, and drive home in silence. The whole thing having been rather awkward. You drop him off, and you head home. You have a big day tomorrow. Sam is coming over bright and early to meet your father so you can both tell him you're getting married.