Pon Farr is the most convenient plot device for sex. TF2 doesn't have pon farr, so I had to use alcohol.

Also, this is my first attempt at smut...

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In a sudden moment of sobriety, the thought "Now how'd I end up like this?" crossed
Sniper's mind. It was quickly shoved aside to make room for better, more productive
thoughts like, "Oh! That's nice" and "Ahh... right there..." as Demoman continued to
suck his cock.

The answer to the one sober thought was simple story of Sniper and Demo wanting to
drink to celebrate some some victory, Soldier telling them the alcohol would make
them weak-minded in tomorrow's battle, and Demo promising the good stuff in his
room. This led to a tipsy "Who Has the Coolest Battle Scar" contest, which caused
Sniper to moon Demo to show off a particularly embarrassing one, and this caused
Demo to discover that Sniper had a ticklish butt. A small tickle fight erupted.
Sniper's pants, which had already been partially off, were now half-off. With all
the contact, Demo getting prettier the more he drank, and them some how being
tangled together on the floor, Sniper was feeling a bit aroused. And showing it.

Perhaps under other circumstances, they would have both instantly sobered up, gotten
off the floor and pretended the whole thing never happened, but such circumstances
were not present. Instead, Demo thought it was kind of hilarious and decided to play
with his companion's starting erection. The proper response was likely to push the
darker man off of him. Of course, that's not what Sniper did, instead he moaned and
awkwardly rubbed his leg against Demo's own forming erection.

That certainly should not have turned into them seeing who could take off the
other's clothing faster. At this point, Demo continuing to tease Sniper's cock was
to be expected- incidentally who said the Scot could be so good with his hands? Sure
his job required perfectly steady hands, so he must have had pretty good motor
control, but chemicals and penises are two different things and being good with one
shouldn't make you good with the other. When Demo started using his mouth, Sniper
stopped thinking about what the Scot should and shouldn't be good at.

It did occur to the Aussie that it wasn't fair for him to be receiving all the
attention. He pushed Demo off of him, with a bit of protest, and repositioned
himself so he could return the gesture. With the two men now sucking on each other,
it seemed inevitable that this become a "Who can make the other come first" contest.
Of course Demo won, he had the time advantage. Sniper let out a squeak from his
throat as he focused on not clenching his teeth while he came. Demo seemed to like
the noise and didn't take too much persuading to reach his own climax.

The two lay awkwardly on the floor after that, face-to-crotch. Demo took the
initiative and moved so they could lay awkwardly on the floor face-to-face. Neither
felt motivated to move so they spent the night on Demo's floor. The next day they'd
discover that Soldier was full of it when he said they'd be weaker if they drank.
They were just as capable as they'd ever been.

Alright, that did explain how Sniper ended up in Demo's room the first encounter,
but it didn't account for this time...