"I'm not sure, but I think the aim of 'Advancing equality of opportunity for women' is one of the exceptions to the EA which allows an event to be women only."
You're right. This comes under Section 158, positive action. If Plymouth University reasonably believes, for example, that women face disadvantage in pursuing careers in marine science, as reflected in their low participation in the field and/or low levels of seniority, then Section 158 allows them to run things like women-only events to encourage greater participation and increase the number of women who apply.
However, there must be at least some sort of rationale for this. Maybe very few women apply to Plymouth compared to men?
However, an article from 2024 states that between 2019 and 2021 there were 9,565 applications for marine science degrees in the UK through UCAS. This was split 52.5% women and 47.5% men. Of those who were accepted, the split was 53.7% women and 46.3% men.
So, certainly when it comes to taking up marine science degrees, women don't appear to be under participating in any way.
https://www.nature.com/articles/s43247-024-01579-1
So there doesn't really seem to be a rationale for Plymouth doing this. If anything, there is a rationale for having an open day for people of different ethnicities. Apparently 92% of applicants for marine science degrees in the UK describe themselves as white.
Where there is a difference is in senior positions in the field. Women are a lot less likely to be in a senior position in research or other employment in this field.
But in that situation it would be a lot more appropriate for whichever body is employing senior people in marine science to undertake positive action to encourage more women into senior positions.
.
Enitrely separately, inviting trans-identifying men to attend as well, they must reasonably believe that trans-identifying men under participate in marine science. I have no idea how they would be able to reach that belief.