The thing is that in truth there's no real right or wrong answer, and depending on where you post you will get a different set of opinions.
If the OP had for instance fallen pregnant and had a baby with this man (which was unplanned) posters would be saying that the son essentially just needs to accept that this is it now.
If she'd got married to him and they'd moved in together there would be an assumption that he needs to be a bit more respectful, especially if he's seventeen and old enough to know better.
And nowhere does anyone on e.g. The relationships board advocate someone who has left a previous relationship spending the rest of their life alone because the kids might not like it.
There has to be middle ground between the kids having all the say in whether the parent is allowed to move on and the parent moving on with no regard what so ever for the children.
It's not unreasonable for the OP to want her children to meet her partner of two years, or even the man she's been in a relationship with and hopes to build a future with at some point. Equally the children are not unreasonable to not want to meet him or if they do, to not like him. There needs to be compromise in the middle somewhere which states that a separated parent does have the right to move on with their life, that the children don't like it, but that mutual respect is something which should be applied from both sides.
And yes, people are very quick to jump to the "he's seventeen and still a child," line when wanting to justify a position bbut if he e.g. Committed a crime or did something illegal or wanted to go abroad and travel the world his right as an almost adult would be upheld by those same people.
Incidentally, how long do people consider that the family home should be maintained after the children go to Uni? I stayed here after I split from my ex because I wanted my DS to have an equal relationship with both of us. When he leaves for university I won't be able to stay here any more unless I have a job, something which atm is looking increasingly unlikely due to various circumstances. Should I be permitted to sell this house and move somewhere cheaper out of London? Even to where my DP lives? Or should this home be maintained at all costs in case DS wants to come back and live here? Or is it ok to buy a different house in a different town on the basis that he can live there too if he wants - if he can travel independently to university he can travel back to the new town then as well, no?