This is a complicated question. You should first of all not think that sexuality is black and white, true or false, right or wrong. Sexuality is fluent; sexuality is complicated; sexuality is messy and sometimes outright frustrating.
Maybe a woman can have been straight all her life and only had lovers and partners who have been male, all up till the day she met the woman who came up to her and said “uh you are cute as a cupcake, lets date,” and from then on they lived happily forever after.
There is no scientific proof that a girl born is bisexual; she will like same-sex and heterosexuality at the same time. However, there is also no basis to prove that a straight girl will become a lesbian. Love is a very sacred thing; no one can go and suspicion it.
But you can choose to believe two people can be in love without labels. Some people are straight in every way but who have had a deep love of a certain someone of the same sex. Take the 1991 film, Fried Green Tomatoes. It recounts two women, Ruth and Idgie, who form a life-long bond. Are they lesbian? The film is coy about that. Under oath, Ruth says of Idgie, “She’s the best friend I ever had, and I love her.”
Maybe you are a heterosexual woman, you fell in love with a man in the past, and you are married. In the few years you have been together, you have always believed that you are a straight woman, you love this man until you meet her. You are deeply attracted to her. At first, you thought that it was just a simple appreciation, but you gradually found that you love this woman, you want to be with her as a lover, you love her not only because of sex. You fall in love with someone because she can be any identity.
A person’s sexual orientation is changed by the external environment or by their factors. Not all straight women can become lesbians, but straight women are likely to become lesbians.
Sexual orientation may change through time, but it is also possible that you are born to be a lesbian, and you have been deliberately avoiding it in the past. The calculus of the heart is a tricky thing, and slapping on labels might feel good, but the labyrinth is very nuanced.
Yes, it is possible for a straight person to love someone of the same sex deeply.