I thought this was a very interesting study, however I have some questions/comments:

The authors might have investigated more what aspects of gender/relationship status could have driven this effect on walking speed

a) The gender effect might have been height related but this does not seem to have been a covariate
b)However, the absence of an effect with a non-romantic partner of the opposite gender seems to suggest that other factors might be at play - were the romantic partners talking more - i.e. a dual task condition?
c)Did they show any public displays of affection beyond those included as study variables - e.g. kissing or wrapping an arm around their girlfriend's waist? Both of these are likely to reduce walking speed.
d) Sexual arousal/an erection might physiologically impede walking gait as well
e) Were any of the women wearing high heels?
f) If they weren't wearing high heels during the study session, do they usually do so when walking with their partner? Men might change their pace more if their girlfriend regularly wear heels, since they are likely to reduce her walking pace

Regardless of these possible issues of confounding and generalisability, it is an interesting study and one I'm glad to see in the literature

Dr Max Barnish