So you made sure the relationship will hang in the balance of this one single game?
Should I let my girlfriend beat me in a chess tournament?

Should I let my girlfriend beat me in a chess tournament?
No way, smash her to pieces... show her who da man.

So I was entering a chess tournament tommorow, and my girlfriend felt bored and wanted to join. Only problem is that she is a complete begginer and doesn't know any tactics.
The worst part is that I'm actually against her tommorow. Should I purposely let her beat me, or should I play an honest game? If so, how can I lose without it looking intentional?
Next time, ask if the TD will try to avoid pairing you with her. In events I run, if the turnout is good enough, I will try and not pair family members in the early rounds, especially if requested, and would do the same for someone in your situation.
Not all TDs do that but it is allowed and doesn't hurt to ask. Unless it is a round robin, then you have no choice :D
Win the game, but don't win either too quickly (which might make her feel stupid) or too slowly (like you're just torturing her).
Of course... this assumes she isn't the next Capablanca!
Play honest. I wouldn't deliberately let a female beat me in chess even if its Helen of troy. ;)