If someone is familiar with Thomistic ethics, this is not a surprise. If the reason that you use a condom is to prevent disease, and not for prevention of conception, it is permissible. Thomistic ethics does not consider secondary factors or secondary consequences. For instance, it does not consider the issue of homosexuality in this decision, but only the prevention of disease. It is a clever way of dealing with moral issues, but it is also the reason that Thomistic ethics is strongly criticized.
That is very much the heart of Catholicism and allows for changing doctrine ..That is how one can have homosexual priests and still teach that homosexuality is a damning sin
St. Thomas, in the Prima Secundae (the first part of the second part) of the Summa Theologica notes in question seven that circumstances (that is secondary factors) are a component part of all moral actions:
That circumstances must be considered, at least so far as St. Thomas and those who follow his lead go, is clearly established in the respondeo of the second article of this question:
Circumstances come under the consideration of the theologian, for a threefold reason. First, because the theologian considers human acts, inasmuch as man is thereby directed to Happiness. Now, everything that is directed to an end should be proportionate to that end. But acts are made proportionate to an end by means of a certain commensurateness, which results from the due circumstances. Hence the theologian has to consider the circumstances. Secondly, because the theologian considers human acts according as they are found to be good or evil, better or worse: and this diversity depends on circumstances, as we shall see further on (18, A10,11; 73, 7). Thirdly, because the theologian considers human acts under the aspect of merit and demerit, which is proper to human acts; and for this it is requisite that they be voluntary. Now a human act is deemed to be voluntary or involuntary, according to knowledge or ignorance of circumstances, as stated above (Question 6, Article 8). Therefore the theologian has to consider circumstances.
http://www.newadvent.org/summa/2007.htm
While it may be stated that Thomas does not hold that a circumstance as a circumstance never makes an argument good or bad, his reason is not because secondary things are not considered. Rather, things that are intrinsically disordered do not fall under circumstances, though a person might mistakenly think that they are only a circumstance, but rather become the object. They relevant analysis is eleven questions later, in question 18, article 10, in the ad secunda:
A circumstance, so long as it is but a circumstance, does not specify an action, since thus it is a mere accident: but when it becomes a principal condition of the object, then it does specify the action.
http://www.newadvent.org/summa/2018.htm#article10
The moral sophistry that is attributed to Thomistic ethics in the above post cannot be derived from Thomas. Those who wish to criticize Thomas on these grounds know not what they do. (If one wants to criticize Thomas, one might begin by quoting him).