Just an update on this. It's never simple!
It is legal to embed the content, as the content is not hosted by the site. The owner of the copyright material has the option to prevent embedding of content when uploading their content to YouTube - so it is the responsibility of the content owner to ensure that any restrictions they wish to place on their content, that they do that.
Ideally if you wish to post content from YouTube, that you only provide the link / URL to the content. and provide the original author and resource location credit, e.g SPFL via YouTube
It is against copyright laws to embed an infringing YouTube video in the forum, ie content not posted by the SPFL on YouTube. Such video is also not legal on YouTube.
The YouTube terms of service says the following:
This clearly states that the owner of the video grants a limited license to embed the video simply by leaving the embed option on (which is part of the functionality if enabled by a feature of the YouTube Service).