A platonic friendship is a close and affectionate relationship between two people that does not involve romantic or sexual feelings. It is based on mutual respect, trust, and support. Full definition
Sex and relationship variables as predictors of sexual attraction in cross-sex platonic friendships between young heterosexual adults. (scienceofrelationships.com)
«The new infidelity is between people who unwittingly form deep, passionate connections before realizing that they've crossed the line from platonic friendship into romantic love. (thespruce.com)
And if you trust your spouse / partner (and s / he trusts you), it is assumed that you both can maintain a purely platonic friendship with someone of the opposite sex (or someone of the same sex if you're gay). (strengthenyourrelationship.com)