If you poke around the internet, there are a lot of fan-made Portal valentines to be found. But it's pretty hard to beat the official printable Portal Valentine's Day cards from Valve, released two years ago, in 2011.
There are six cards to be found on this free download (PDF), featuring GLaDOS, Chell, sentry turrets, a weighted companion cube and an intelligence sphere.
If you prefer something that can be emailed, Tumblr user Tealgeezus has created some adorable animated gifentines, featuring GLaDOS, an intelligence sphere, a sentry turret, a weighted companion cube, the space sphere and Cave Johnson's combustible lemon.
(Credit: Gourmet Gaming)
And if none of these options quite floats your Portal love boat, why not show you care with a home-made, deliciously unambiguous Portal devil's food cake? Actually, why not a full, video game-themed three-course meal? We'd probably steer clear of the fluffy mackerel pudding, though.