Ever since using Woohooer I noticed that couples in a romantic relationship now come with their gender preference already set to the sex of their partner (with low values like -1/2) when I start a new game. At least I suspect that this is Woohooer's doing in order to enable autonomous Woohoo go smoothly.

The reason why I am posting this in Issues is that this mechanism fails when traveling to WA destinations. The locals' preference remains at 0/0 and this, surprisingly, causes lag.
I tested this in various games and watched married local couples in France, for example, have the bed icon in their thought bubbles every 5-10 Sim minutes, while remaining immobile and causing the game to slow down. So, I guess they wanted to Woohoo but failed. After I set their gender preference towards their partners with MC they then immediately headed for the beds and gameplay became smooth again.
As Woohooer seems to set gender preference on every new load-up of a main world, is there a possibility for it to do the same when loading a travel world for the first time?

Actually that's the game that sets it not Woohooer why it doesn't set gender preference for sims in the WA worlds is they where never intended to be autonomously romantic and breed on their own. The game would simply clone them if another generation was ever needed.
The Character Import XML in the world files is where it's set.

I've never noticed the lag effect, but what I find annoying is that often couples in newly spawned worlds don't know each other very well or don't have very high relationship scores towards each other (they don't like each other enough yet). Sometimes I go in and change those instead, maybe not all of them since of course not every couple in the world is a happy one, and I might bump up the base chances of romance and woohoo townwide temporarily., Then the gender prefs take care of themselves after a while. Guess we're really accomplishing about the same things though, by making already partnered couples more attractive to each other than they seem to be by default.

I've noticed it in the Nectary in France, which has two double beds, so when two couples are there the pressure seems to be great enough to make it noticeable.
There relationship values must be high enough though, because I've set Woohooer to allow autonomous Woohoo for romantic couples with a score over 60 (which would be the Good Friends threshold otherwise). From my experience they are also reluctant to flirt unless their preference has already been set.
@sittingbear: That's interesting to know, so maybe in the past I had rather the issue that something must have wiped these settings before start-up. BTW, where is this data stored if a world does not contain the Character Import XML? Like Sunset Valley, for example.

Custom worlds will never have one as EA never made the Tool to make the XML available.
As for Sunset Valley that I can only guess is buried deep in the game files in some unreachable area or maybe it's in one the .dlls

Ah right almost forgot but with pre-populated custom worlds if the world creator sets the gender preference while in CAW EIG mode or I think imports them with Porter the sims should retain the gender preference data or it's the lot they live on that holds onto to the data.

Correct, Woohooer has no mechanic in it to set gender preference. The core sets all that up as sittingbear described, or SP does for Sims it immigrates. The vacation worlds do have this data for starting couples and that should be working. I can see how woohooer would hammer when their woohoo cooldown was negative and it couldn't find anyone. It's going to keep looping. If this is causing issues, hit the town with MC / Town / Randomize Undecided Gender Preference.