This is a very interesting approach you have. It certainly carries risks. This opinion is NOT my advice to undertake any approach, to continue what you're doing or to abandon it.
Here's my quick view of the how it plays under the UDRP.
Under the UDRP, the question is whether or not the registration of a non-common word domain name, which is used to list sites of parties who have trademarks for the name, constitutes bad faith registration and use. I think the question is whether or not you are using the name to confuse users looking for the tm site. I agree that your site does not, per se, confuse, but the tm owner may bring up the concept of initial interest confusion -- meaning users are confused for a short time. However, again, you correct that by giving the user the referrer URL. Certainly, however, by putting up those URLs you are waiving any possible argument that you do not have knowledge of the trademark owner, and are allowing them to make the argument that you are, essentially, profiting off their mark. If the tm is a unique mark, like xerox, I think it is clear you can't register xerox.com and redirect to their official site, and make money off of pop-ups. The question you ask is whether it's ok if there's more than one tm holder.
But, then there's the case of ralph-lauren-polo.com which remarkable went in favor of the domain owner, who did not even file a response! Because the respondent had pointed the domain name to Ralph Lauren's web site (the complainant), the panel held that it was not being used in bad faith.
http://arbiter.wipo.int/domains/decisions/html/2002/d2002-0148.html
Nevertheless, I think most panelists would see this as bad faith use, likely finding that you are confusing users to benefit commercially, despite the fact that you are not really confusing anyone in the end. They'll just see it as dirty business, I think, if the name is not a common word.
I would disagree, however. In fact, where there are multiple tm owners, the argument can be made you help everyone since, if the domain were owned by just one of the competing tm owners, they would get all the traffic at the expense of others and the consumers looking for those other sites.
As for legitimate interest, the question will be whether or not you can use a domain name that is not a common word for a bona fide service. I think many panelists would say no. If the domain name, however, is a common word or geographic term, I think there's a good argument you have a legitimate interest.
In conclusion, I agree with those who consider this to be a very risky application. Although very clever, and probably not a violation of the UDRP if it is strictly applied, it will certainly invite trademark holders to attack you. You cannot plead ignorance since your use admits knowledge of the marks. Of course, directories list sites, but it is the domain name that raises the obvious problem. Where a case is brought, Panelists will likely rule in favor of the tm owner where the domain name does not incorporate a common word or term. However, you can always point to the Ralph Lauren case for help.
ONe twist on your approach is to simply have a statement on the sites that invites parties to place a free directory listing on the URL. This has multiple benefits:
1- tm owner consents to use and won't be able to sue you;
2- the tm owners consented use can help you argue bona fide use and, thus, legitimate interest
3- tm owner will be happy with listing and maybe place your URL on its literature increasing your traffic. (along this line, perhaps, you could offer third-level or subdomains.
Just some random thoughts. I cannot recommend any strategy. These all contain substantial risks because of the value of domain names incorporating trademarks to the corresponding tm owners. They will often spend money on litigation to get these valuable assets, and you will be forced to spend money to defend them.
draqon is correct. 
