I honestly feel that at 18 years we as parents cannot really 'force' our way of thinking on our children.
This is also the age of sexual experimentation,
and while your daughter may or may not be gay,
that is in itself NOT something to be unduly worried about since it is a natural state of affairs
She may want to test her adult relationship and boundaries, and then decide about her sexual preferences
Your concern should be directed more towards her physical and mental well being in general, her academic performance, and what / how she wants to move ahead in her life
Remember that while this is an important part of her life, it is NOT what she is all about
So try to be friendly, non - judgmental and offer your opinion as and when solicited.
I know it is very easy for us parents to try to 'guide' the child as per our experiences
but try resisting that urge, and let her find out more about herself, and adult relationships in a complicated world
I hope this makes some sense,
Please remember to accept the answer by pressing the GREEN accept button, do get back for any clarifications