Normally when conducting library research, we focus on constructing "good enough" keyword searches to obtain candidate results. Then we review these results to see how they are classified in the "subject headings", "subjects" or "descriptors" fields (the name differs depending on the database used). Although the term "transnational sexualities" is being used in research circles, it is not (yet) being used to classify scholarly research.
The meaning and application of "transnational sexualities" is wide-ranging. Subject headings may help but they will not provide all the nuances of the topic that you wish to research. For instance, the sociological term for sexual behaviour which is used in the catalogue and some of the databases is
This can be refined by geography: Sex customs -- Canada
It can also denote differences in sex customs across cultures: Sex customs -- Cross-cultural studies.
Some subject headings may include links to broader or narrower terms; consult these to see if they better match what you are seeking.
Here are some other subject headings which may be of interest in the catalogue:
We will be depending primarily on constructing a sequence of keyword searches in order to cover the topic properly.