I'm not sure how i should put this....either
A) Mantis [BLEEP]ed a tree; or
B) Mantis [BLEEP]ed the corpse of her dead boyfriend.
Well, actually, it's kinda both. She [BLEEP]ed the corpse of her dead boyfriend that was re-animated by the tree she married!
See, during the Celestial Madonna epic, we were introduced to the Cotati, a sentient plant species that evolved on the Kree homeworld. Some pacifist Kree kung-fu priests (yes, pacifist Kree kung-fu priests) brought a Cotati to earth millennia ago, and planted him in their Vietnamese temple.
When the tree and humanity were ready...well, let's allow the re-animated corpse of the Swordsman explain everything:
Look, the Swordsman actually died died. Dead. Corpse.
And if I were going to propose to some woman, I don't think offering her the re-animated body of her dead lover would be viewed as a cool "wedding gift." Especially if "mating" with it were involved.
But these are comics, man...
Mantis eventually came back--creator Steve Englehart was so in love his own character he worked her into virtually every single project he ever wrote, including comics for other companies--and yeah, she had a kid. A physical child. Which means...EWWWWWWW!!!!
I kid because I love. The Celestial Madonna epic is really great (oops--SPOILER ALERT!), and everyone should read the whole saga.
But we should never forget as we enjoy the excellent Guardians Of The Galaxy Vol. 2--Mantis married a tree and boinked the corpse of her dead boyfriend.
From Giant-Size Avengers #4 (1975)