Look it's fanfic, maybe they have some kind of advanced healing technology (and maybe the wound wasn't as bad as it looked once it was cleaned and stitched) - but if you expect me to suspend disbelief there, you had better also have the tech to have cleared them of bloodborne pathogens when they showed up in medbay covered in each other's blood.

Or maybe you should be using "gut wound" rather than "no condoms" as your excuse to draw out the UST. That works too.