Not to mention the biologist is in an underground room on an alien planet. Oh, and the biologist and the geologist who is with him just saw a massive pile of dead bodies. And then they were told, over intercom by the ship captain, that some sensor is picking up a sign of life.
Given all of the events surrounding and preceding the encounter with the vagina-snake, you would think the biologist would be like "fuck this" and leave the snake alone. WHY WOULD HE POSSIBLY INTERACT WITH THE VAGINA-SNAKE!?
The two people involved in this scene are: Fifield, The Geologist. And Millburn, The Biologist.
Do you remember the first scene we see them in? Everyone has just woken up from the cryo-sleep. We see people in the dining hall. Millburn sits at the table with Fifield.
"Is this seat taken?" asks Millburn. He sits. "Millburn, biology." He holds out a hand to Fifield. "Nice to meet you."
Fifield drinks his soup. Milburn retracts his hand. Fifield almost whispers, "Look, um... [all or no] thanks, but, uh, I've been asleep for two years. I ain't here to be your friend. I'm here to make money. You got that?"
Millburn nods, says "Okay."
In the mission briefing scene, Fifield, the resident dick, questions Shaw and Holloway. "So you're saying we're here because of a map you two kids found in a cave, is that right?" The camera is on Fifield, but it's a medium shot and in the frame we see another scientist and Millburn. As Fifield speaks, Milburn smiles, amused. Shaw and Fifield go back and forth. After Shaw says Engineers "engineered us," Fifield goes "Bull shit". We see him say it, and we also see how FUNNY Millburn thinks it is. Then Millburn jumps into the conversation. He says, "Okay, so, uh...Do you have anything to back that up? I mean, look, if you're willing to discount three centuries of Darwinism...that's woooo," and gives a sarcastic thumbs up gesture. "Then how do you know? HMM?"
We could just view this scene as two separate instances. Fifield is a dick. Millburn is a dick. Or, we could say: Millburn sat down at Fifield's table for a reason; he laughed at Fifield's jokes for a reason; and there's a reason he's the only one other than Fifield to question Holloway and Shaw. That reason: he wants to be friends with Fifield.
To me, Millburn is displaying classic relationship-building behaviors. Introduction, laughing at what someone else says when no one else is, joining someone in an activity/taking on someone's opinion.
Later, when everyone is gathered around the decapitated Engineer, in the depths of the Engineer base, Fifield gets scared. He throws a fit and declares he's leaving. "Anyone wanna join me, hey?" Then he addresses Millburn. "Are you staying." PEER PRESSURE BRO. Millburn agrees to go with Fifield.
The two are now buddy-buddy.
So what's this have to do with the Vagina-Snake?
I'm arguing while Millburn's actions regarding the vagina-snake may ostensibly seem...moronic...they are, in fact, part of this little drama that's been going on between Millburn and Fifield.
Millburn is trying to impress Fifield.
I think nearly everyone has done something similar. You make a new friend, or you're trying to make a new friend, or there's a girl you like, or a guy you like...and you want to impress them. You brag about something, or you try to show off some skill. How many times have we seen this in a movie or a TV show? A couple is at a fair or Coney Island or an amusement park, and the guy tries to impress the girl by winning her some prize?
Well. Millburn was trying to impress Fifield. And just happened to do so in one of the stupidest ways possible--"hey unknown creature on another planet that looks like a fucking cobra, I'm going to touch you now."
So this is the theory behind WHY IN THE WORLD would Millburn not run away from the vagina-snake. Do I think the scene is good? Not really. But do I think it MAKES NO SENSE AT ALL? No. I think there's clear justification for the action. I just don't know if I buy anyone really wanting to impress someone so much they would choose that scenario, of all scenarios, to try to show off. [insert joke about hubris]