An internet vlogger has stunned viewers with the reason she let her best friend sleep with her boyfriend.

YouTuber Lena Nersesian, who calls herself Lena The Plug, told her 900,000 subscribers she wanted her friend Emily to have sex with Adam so she can “try his penis.”

In an odd twist of logic, she reasoned that as Emily knew everything about her sex life, she should join in.

She said: “Girls and guys share their sexual stories with each other.”

“Friends tell each other everything, so Emily has seen pictures of Adam’s d–k, she knows all about our sex life, I tell her everything.”

“And now, I don’t have to tell her, she can just try his penis.”

The term “oversharing” could have been invented for this moment.

Adam was pretty happy to take part in the online experiment, although he said he was feeling the pressure to “perform.”

He said: “Let me just say, I can’t even wrap my f—–g head around how my life got to the point where I’m engaging in — not only this activity in the first place — but with two girls that look like this.”

The girls are then seen in skimpy outfits twerking for Adam — who has his own YouTube channel — before he joins them on the bed.

After the girls had sex with him — off camera — the threesome discussed their antics in X-rated detail.

Adam said: “That was probably one of the greatest experiences of my life.”

But Lena’s explanation seems to have left some watchers baffled.

One user wrote: “That was just weird, seriously.”

Another added: “Just because you talk about your partner doesn’t mean your mates all need to sleep with them to understand.”

A third added: “Why the f–k are you both even proud, I’m so confused, IS THIS EVEN NORMAL?”

The girls also received a torrent of abuse from viewers, who called them “sluts” and “whores” in the thread.

Before the XXX-rated episode, Lena revealed that it’s not the first time they had “collab’d on a penis.”

She went into intimate detail about an unnamed “A-list rapper” who invited them back to his house before bedding them both in an “awkward” threesome.

Earlier this year, Lena caused a stir when she promised to release a sex tape if she got 1 million subscribers.